I tried upgrading a copy of my Moodle production website from version 1.9.10+ (Build: 20110112) to 2.0.1 (Build: 20101225) and experienced the following problem. I'm not using a more recent version because of the problems reported in MDL-26025 and I'm not sure if this problem is related to that, but these are the crash details:

Initially the beginning of the upgrade went fine until I got to a screen saying "Migrated user icons 97/97" (see attached screenshot). The upgrade seemed to "hang" at this point, I left it for about two and a half hours and there was still no progress.

Since this was a test upgrade I eventually pressed the F5 key to refresh the page and see what happens. The installation seemed to continue and I got several "Success" messages as each of the tables was upgraded.

Eventually I got a "Error reading from database" message on the enrol_category table. The details show the dreaded "The ntext data type cannot be selected as DISTINCT because it is not comparable" error:

System
Updating tags itemtype
Success

mod_assignment
Migrating assignment submissions - 8651/8651.
0.13 sec

100% Success

mod_chat
Success

mod_choice
Success

mod_data
Success

mod_feedback
Success

mod_folder
Success

mod_forum
Success

mod_glossary
Success

mod_imscp
Success

mod_label
Success

mod_lesson
Migrating lesson files - 2/2.

100% Success

mod_page
Success

mod_quiz
Success

mod_resource
Success

mod_scorm
Success

mod_survey
Success

mod_url
Success

mod_wiki
Adding new fields to wiki table
Renaming old wiki module tables
Installing new wiki module tables
Migrating old wikis to new wikis
Migrating old entries to new subwikis
Migrating old pages to new pages
Migrating old history to new history
Caching content
Cleaning wiki table
Updating comments and tags
Updating tags itemtype
Success

mod_workshop
Renaming old workshop module tables
Preparing new workshop module tables
Copying workshop core data
Copying submissions

Copying assessments
Success

auth_cas
Success

auth_db
Success

auth_fc
Success

auth_imap
Success

auth_ldap
Success

auth_mnet
Success

auth_nntp
Success

auth_pam
Success

auth_pop3
Success

auth_radius
Success

auth_shibboleth
Success

enrol_authorize
Success

enrol_category
Error reading from database

More information about this error
Debug info: SQLState: 42000<br>
Error Code: 421<br>
Message: [Microsoft][SQL Server Native Client 10.0][SQL Server]The ntext data type cannot be selected as DISTINCT because it is not comparable.<br>
SQLState: 42000<br>
Error Code: 421<br>
Message: [Microsoft][SQL Server Native Client 10.0][SQL Server]The ntext data type cannot be selected as DISTINCT because it is not comparable.<br>

SELECT DISTINCT c.*
FROM mdl_course c
JOIN mdl_context ctx ON (ctx.instanceid = c.id AND ctx.contextlevel = '50')
JOIN (SELECT DISTINCT cctx.path
FROM mdl_course_categories cc
JOIN mdl_context cctx ON (cctx.instanceid = cc.id AND cctx.contextlevel = '40')
JOIN mdl_role_assignments ra ON (ra.contextid = cctx.id AND ra.roleid IN ('12','4'))
) cat ON (ctx.path LIKE CAST(cat.path AS VARCHAR(255)) + CAST('/%' AS VARCHAR(255)) )
LEFT JOIN mdl_enrol e ON (e.courseid = c.id AND e.enrol = 'category')
WHERE e.id IS NULL
[array (
0 => 50,
1 => 40,
2 => 12,
3 => 4,
)]
Stack trace:
•line 391 of \lib\dml\moodle_database.php: dml_read_exception thrown
•line 252 of \lib\dml\sqlsrv_native_moodle_database.php: call to moodle_database->query_end()
•line 363 of \lib\dml\sqlsrv_native_moodle_database.php: call to sqlsrv_native_moodle_database->query_end()
•line 761 of \lib\dml\sqlsrv_native_moodle_database.php: call to sqlsrv_native_moodle_database->do_query()
•line 276 of \enrol\category\locallib.php: call to sqlsrv_native_moodle_database->get_recordset_sql()
•line 72 of \enrol\category\db\install.php: call to enrol_category_sync_full()
•line 335 of \lib\upgradelib.php: call to xmldb_enrol_category_install()
•line 1352 of \lib\upgradelib.php: call to upgrade_plugins()
•line 290 of \admin\index.php: call to upgrade_noncore()

I clicked the Continue button and I was taken to the Moodle Front Page. The upgrade seems to have completed! I can see the Site News and my Courses. I then login as Adminsitrator and Moodle displays the Plugins Check screen. There are no 'missing' blocks or activities or filters. At the bottom of the page is a Upgrade button. I click on it.

After a few minutes I get a list of tables that are successfully upgraded, so the upgrade seems to be continuing:

enrol_cohort
Success

enrol_database
Success

enrol_flatfile
Success

enrol_guest
Success

enrol_imsenterprise
Success

enrol_ldap
Success

enrol_manual
Success

enrol_meta
Success

enrol_mnet
Success

enrol_paypal
Success

enrol_self
Success

message_email
Success

message_jabber
Success

message_popup
Success

block_blog_menu
Success

block_blog_recent
Success

block_comments
Success

block_community
Success

block_completionstatus
Success

block_course_overview
Success

block_feedback
Success

block_html
Success

block_mnet_hosts
Success

block_myprofile
Success

block_navigation
Success

block_private_files
Success

block_rss_client
Success

block_search
Success

block_selfcompletion
Success

block_settings
Success

block_tags
Success

filter_emoticon
Success

filter_mediaplugin
Success

filter_tex
Success

filter_urltolink
Success

editor_textarea
Success

editor_tinymce
Success

report_courseoverview
Success

report_customlang
Success

report_questioninstances
Success

report_security
Success

report_unittest
Success

report_unsuproles
Success

coursereport_completion
Success

coursereport_log
Success

coursereport_outline
Success

coursereport_participation
Success

coursereport_progress
Success

coursereport_stats
Success

gradeexport_ods
Success

gradeexport_txt
Success

gradeexport_xls
Success

gradeexport_xml
Success

gradeimport_csv
Success

gradeimport_xml
Success

gradereport_grader
Success

gradereport_outcomes
Success

gradereport_overview
Success

gradereport_user
Success

mnetservice_enrol
Success

webservice_amf
Success

webservice_rest
Success

webservice_soap
Success

webservice_xmlrpc
Success

repository_alfresco
Success

repository_boxnet
Success

repository_coursefiles
Success

repository_dropbox
Success

repository_filesystem
Success

repository_flickr
Success

repository_flickr_public
Success

repository_googledocs
Success

repository_local
Success

repository_merlot
Success

repository_picasa
Success

repository_recent
Success

repository_s3
Success

repository_upload
Success

repository_url
Success

repository_user
Success

repository_webdav
Success

repository_wikimedia
Success

repository_youtube
Success

portfolio_boxnet
Success

portfolio_download
Success

portfolio_flickr
Success

portfolio_googledocs
Success

portfolio_mahara
Success

portfolio_picasa
Success

qtype_calculated
Success

qtype_essay
Success

qtype_match
Success

qtype_multianswer
Success

qtype_multichoice
Success

qtype_numerical
Success

qtype_randomsamatch
Success

qtype_shortanswer
Success

qtype_truefalse
Success

assignment_online
Success

quiz_overview
Success

quiz_statistics
Success

workshopform_accumulative
Success

workshopform_comments
Success

workshopform_numerrors
Success

workshopform_rubric
Success

workshopeval_best
Success

At the bottom of the page is a Continue button. I click on it and I am taken to the "New settings" screens. I leave all the defaults and click the Save Changes button.

Moodle displays the "The cron.php maintenance script has not been run for at least 24 hours" message. This is nornal since I have disabled the cron job.

Now the upgrade seems to have completed and I don't see any problems in my courses - but I have over 150 courses and I haven't yet tested each one thoroughly).

I am concerned about the above SQL errors that I did get and that I had to "restart" the upgrade half-way through. These errors imply that my database was not fully upgraded to the Moodle 2 structure and this could cause problems in the future. (So I'm not willing to upgrade my production instance of Moodle just yet.)

I'm not sure if the fixes in MDL-26025 will resolve these problems, but I felt the need to report the above!