by Oliver Smitkowski.
I am a colleague of Jens. We have since found out that the problem is related to the customfield_multiselect plugin. We use this plugin to assign institutions to our courses. Some courses are offered jointly by several institutions.
With the plugin the error "Argument #1 ($string) must be of type string, array given" occurs with an array that contains only one element, the number 0. So here, moodle tries to backup a course that is offered by a single institution, namely the first from the list of options.
The backups run through if we deactivate the plugin (or with the fix described above).
I will also try to contact the developers of the plugin, but I assume that the problem is not with the plugin alone.