@Alex T⚜️ answered to my support request and gave me some hints on the error messages in my sys_transcoder.log file. I assumed that in my case deletion of files in the S3 bucket occurs because I have two UNA framework instances running on the same server. One in the root folder of my server and one in a subfolder (this was recommended long time ago to have the option to use premium modules with one single UNA license), which is my test environment before going live with new modules. Alex confirmed my assumption that two instances have to have two seperate buckets. After that I added a new bucket for my test server and since that I'm observing.

Alex told me further that resized pictures expire after 30 days. But when they are called up again, they should be created on the fly from the original uploaded pictures. But in my case it seems that I've lost some of those original pictures, maybe caused by the shared single bucket for two UNA instances.

Alex told me, we'll wait until other UNA framework users report similar behavior. Well, @Alex T⚜️ , at least another case.

The S3 buckets on the IONOS cloud servers seem to operate without issues. Their support staff could even tell me how many object deletion instructions were sent over the UNA framework API. The S3 buckets are not test versions, as far as I know. They are ready for production. IONOS once charged me. I think it was 3 cents for a month. After that they stopped charging, because I think expenses for charging exceeded profits by far. :-)

Thanks for your comprehensive answer. In fact, it should also be possible during the test period to run with 2 installations on a server (shared hosting), where you have to use 2 separate buckets. I'm will test it again.