phred, thanks for your answer.
I found the MP3-rating set to ID3v2.3 in the tag prereferences. The "also save ID3v1 tags" is checked.
Since I do not really know much about these standards (ID3v2.3 or ID3v2.4 or APEv2 etc.) I wonder, if my settings are correct.
https://imgur.com/a/3gzGDotMaybe someone else can confirm if the MP3 ratings done in MB are displayed in iOS music players or not?
I have got a pretty big music library with about 30.000 files. So re-rating them all manually after copying or streaming to my iPad is not really an option.