getting cocoAspell (2.5) to work under macOS 11.5.2

I would like to use aspell dictioniaries for spellchecking, e.g. in Mail.app (macOS 11.5.2). I didn't use any external spell checker befor on any macOS. So it's the first time I'm dealing with cocoAspell here.

I can install cocoAspell 2.5 from the DMG. An icon appears in Systemsettings->Spelling and I can choose (tic) dictionaries. I can also see which .rws file is being used.

Later, when writing an email in Mail.app pressing CMD+SHIFT: I can see the selected dictionaries in the dialog that pops up on the key combination.

The selected dictionaries appear with a suffix "(Aspell)" in the dialog box. Choosing one of thes Aspell-dictionaries results in the spell checking dialog not functioning. No words list appears nor is the advance to next button functioning.