Cannot import slides

I just installed Articulate 360 yesterday (so it is up to date) and tried to import a 2021 storyline course in order to update it to our new template. Everything looked fine during the import, except that no slides were imported. There was no error message, just a blank scene. A colleague of mine also tried to import this file with no success.

I also tried to copy and paste slides, but that did not work either. At first I tried to copy/ paste an entire scene, but it did not work. Then I copied multiple slides, that did not work. Then I copied the first slide and it DID work. However, trying to do individual slides after that it did not work.

I have no problem importing other courses.

The course that I am not able to import works just fine when previewed. It consists of both regular slides and has quiz slides. The course is on my C drive, not a network drive. 

Any thoughts on how to bring the course into our new template?