Can't backup the Whatsapp chats to my iCloud

I can't backup Whatsapp chats to my iCloud (1.6 GB in use of 5 GB). It has worked well for years. I have owned an iPhone 14 Pro for several weeks and from that moment on it is no longer possible to make a backup. I can't solve it. The "Backup chats" screen in Whatsapp shows a spinning cursor at "Backing up...Preparing". The total size of the latest backup is from December 7, 2022 and is 1.19 GB in size.

Apple Support Europe has been working on it for hours and can't explain let alone fix the backup problem. Everything is going well on the Apple side, it is said, after many tests.

Apple Support Europe has called me again a few times in the last few days to ask if the backup problem to iCloud is still current. Of course I had to admit honestly that the problem is still not solved. At the request of Apple Support, I increased my iCloud plan from 5GB to 50GB. However, I still can't backup to iCloud from Whatsapp app.

Whatsapp Support has no clue either, but they did let me know that this issue has been escalated to the appropriate parties and they are working to get this fixed.