We appear to have identified an issue causing a sessionwaiterr error code on a course restore.

Timed out while waiting for session lock.
Wait for you current requests to finish and try again later

Error code: sessionwaiterr

• line 1365 of \lib\dml\sqlsrv_native_moodle_database.php: dml_sessionwait_exception thrown
• line 178 of \lib\classes\session\database.php: call to sqlsrv_native_moodle_database->get_session_lock()
• line ? of unknownfile: call to core\session\database->handler_read()
• line 42 of \lib\classes\session\handler.php: call to session_start()
• line 77 of \lib\classes\session\manager.php: call to core\session\handler->start()
• line 806 of \lib\setup.php: call to core\session\manager::start()
• line 711 of \config.php: call to require_once()
• line 31 of \index.php: call to require_once()

Once the error appears, the only way to clear it is to restart the site in IIS.

We are running Moodle 2.6+ with PHP 5.5.6 on MSSQL 2008 R2 and IIS 7.5, and we are using Microsoft Driver for PHP.

The issue is with all course restores (of any size backup) and happens in every browser.
Walking through the course restore process, the course begins restoring but never finishes.
After waiting some time the error above appears, either on screen or in php error logs.

I can see that the course is partially created up to about section 1 before it stops and locks up. Deleting the course also takes some time, but hasn't failed so far.