Cannot upgrade to CoursePress 2: Update unsuccessful. Please try again!

Hello.
I made a copy of the site that I want to upgrade to CoursePress 2, so I can test things out. So far I wasn't even able to upgrade. It says: Update unsuccessful. Please try again!
I tried with old and new PHP versions, and with plugins deactivated.
At this point I need your help. I granted support access, you can try it. Let me know if you need anything from me.
Regards.