Tim Hunt

This is a problem that happens sometimes, and is almost certainly no related to any of the plugins. You will probably find more info if you search for the error message in the tracker.

Exactly what the restore code is trying to do is a bit complex. It's when it thinks that a question in the backup file matches one in the place you are resotring too, and then cannot match up the details. (That is: "I think this multi-choice question in the backup file should be the same as this one in the course I am restoring to, except that that in the backup file, one of the choices is 'die Bildauflosung', but the question in the target course does not see to have that choice".)

You probably need to look at the gradebook database tables.

Or, rather that looking at the DB schema, perhaps looks at the list of web services, and the grade-related ones? https://docs.moodle.org/dev/Web_service_API_functions

That string does exist https://github.com/moodle/moodle/blob/master/lang/en/question.php#L444

Moodle cannot create words that have never been input into it. They must come from somewhere. (Perhaps try a full-text search in the database to see if you can see where they come from?)

Generally, it's not a good idea to be using 3.9.0. There have been 10 bug-fix releases since then on the 3.9 branch (https://docs.moodle.org/dev/Releases#Moodle_3.9_.28LTS.29) - however, I don't recall any bug like this.

Read through https://moodle.org/security/ and make your own decisions.

The sensible thing to do is to get your systems and processes worked out, so that you can alwasy upgrade your Moodle when a new version comes out.