by Erackson Brito.
So, while I was writing the first response, I downloaded the spirit of guessing and spotted the 'restore_course_defaultcustom_setting->validate_value()' and thought that that content number could be somewhere in the backup file. So I made a file search for that number and figured it was set as timestamp for startdate. It points, though, to Saturday, 30 July 2050 02:00:00. So I thought that would be the problem and changed it in the DB, manually backed up the course and tried restoring it again. It worked!
Don't ask me how it got that messed, just believe me when I say this value came with the dump files my client sent me.
Anyway, thanks, Ken, for your time and 'spirit of sharing'.
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'spirit of guessing', Erackson