Hi Fabian
Your course probably has a H5P activity in it with a content type (interactive video) not supported by Totara.
We have a recent fix TL-40907 to upgrade the H5P activity in Totara 18.9 so upgrading to this version should resolve this.
If you aren't ready for upgrading then backing up without the H5P activity should let you continue with the course (without the problem activity) ad the H5P can be restored when you have upgraded.
regards