According to the error screen, looks like it's happening not at the end.
"found a workaround restoring courses"
and what work-around was that?
Other than version of moodle (3.11) might help to know versions of PHP + PHP extensions, and MariaDB/MySQL DB version ... assuming linux.
And it also wouldn't hurt to share where site is hosted (provider?) as well as what package leased ... if remotely hosted.
The course table in a 4.1. highest (don't have a 3.11 anymore) shows:
| Field | Type | Null | Key | Default | Extra |
| showactivitydates | tinyint(1) | NO | | 0 | |
Values are either a 0 or a 1.
So before you backup a course, go into it and check the settings of the course.
Look at 'Show Activity Dates' ... it's either a No or Yes there.
'SoS', Ken