"unknown error" - only in CustomGPT while uploading files vom ChatUser

Strange problem:
My CustomGPT prompts the user to upload a small CSV file to analyse the data it contains. This is structured as follows:
Index,Profile,Status,CallId,DateTimeCall,Caller,DateTimeEvent,TypeOfEvent,MfVNumber,UserName,UserCallNumber,

If I do this in the normal chat window (outside a CustomGPT), it can be analysed without any problems.

However, it does not work within the CustomGPT!
Even when uploading, either ‘An unknown error has occurred’ appears in a red message box OR there is even a preview (named ‘Table’) in the chat window with the correct values - BUT then the chat prompts you to upload a file again because it supposedly hasn’t received one.

Does anyone have an idea why this happens?
The file contains only a few test data and is only 1812 bytes in size. All data is also ‘valid’.

How can this be prevented?

Many thanks for a tip!