Urgent: Unable to replace recording for 3 upcoming streamed sessions (500 Error)

I have three sessions scheduled for next week as streamed webinars and I urgently need to replace the uploaded recording. I have done this many times in the past without any issues, but now I constantly receive a PUT 500 Internal Server Error whenever I try to swap the video.
Almost 1,000 people are registered across the three sessions, so this is extremely time-sensitive. I’m already in contact with GoTo Support, but I haven’t received a solution yet.
I tested it with a separate dummy session, and there the video replacement worked without any problems. So the issue seems to affect only the three scheduled webinars with existing registrations.
Does anyone know what could cause this or whether I really need to create three completely new sessions and manually re-add all registrants?
Any help would be greatly appreciated. It’s urgent.

Thank you so much!