Moodle 3.8.1 IMS Cartridge backup fails

Moodle 3.8.1 on PHP-FPM

Postgresql 10.6 DB on separate server

1. Create a course and add a file resource

2. Backup a course as IMS Common Cartridge (course settings -> backup -> click IMS Common Cartridge 1.1)

3. Click Jump to final step

After a few seconds, backup fails and php-fpm.log shows:

[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "NOTICE: PHP message: PHP Warning: copy(/mnt/moodledata/temp/backup/efaf8007d11640cddaf4a24c77fc207e/files/7b/7b0c9d064cda3b610347dc27c8588981a3c5c014): failed to open stream: No such file or directory in /opt/moodle/backup/cc/cc_lib/cc_utils.php on line 181"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "NOTICE: PHP message: Default exception handler: Exception - Unable to copy files! Debug: "
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "Error code: generalexceptionmessage"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 182 of /backup/cc/cc_lib/cc_utils.php: RuntimeException thrown"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 218 of /backup/cc/cc_lib/cc_utils.php: call to cc_helpers::add_files()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 40 of /backup/cc/cc_lib/cc_converter_resource.php: call to cc_helpers::handle_resource_content()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 168 of /backup/cc/cc_lib/cc_convert_moodle2.php: call to cc_converter_resource->convert()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 121 of /backup/cc/cc_lib/cc_convert_moodle2.php: call to cc_convert_moodle2::process_sequence()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 146 of /backup/converter/imscc11/backuplib.php: call to cc_convert_moodle2::convert()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 34 of /backup/util/plan/backup_execution_step.class.php: call to imscc11_backup_convert->define_execution()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 181 of /backup/util/plan/base_task.class.php: call to backup_execution_step->execute()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 178 of /backup/util/plan/base_plan.class.php: call to base_task->execute()"
[04-Feb-2020 16:06:05] WARNING: [pool moodle] child 1432 said into stderr: "* line 119 of /backup/util/plan/backup_plan.class.php: cal..."
[04-Feb-2020 16:06:06] WARNING: [pool moodle] child 1432 said into stderr: "NOTICE: PHP message: Potential coding error - existing temptables found when disposing database. Must be dropped!"

I found a work-around at Step 2.

3) Instead of clicking Jump to final step, click Next

All the checkboxes down the left side are grey, but checked.  All of the icons down the right side are red x's and lock icons. Scroll to bottom.  Click Next.

Screen with Filename and Backup settings - can't do anything here ... Click Previous.

Click (Show type options).  For each line, click All (all checkboxes down left side are accessible and checked).  Scroll to bottom.  Click Next.

Screen with Filename and Backup settings - all checkboxes on left are green.  Scroll to bottom and click Perform Backup.  

Screen reports The backup file was successfully created.  Click Continue.

One oddity:

Under User private backup area, an .imscc file displays.  This backup is large ... 400+MB.

Click Manage backup files.  Weirdly, the icon is the blue H5P icon.

Also, the backup icon is H3P