Hi guys, thank you for your help.
Actually the solution was way easier than expected.
The problem is simply that OpenAI states the maximum size being 25 MB which seems to be false as 20 MB also resulted in the script crashing as soon as a file of that size or more got through the chunking process.
Changing the two 20’s into 10’s actually solved the problem and now even 3 hour podcasts of 300MB or more can be done with easy and the script runs without any problems.
Thank you again for your help, the thread can be closed now 