Thanks for your response.
I tried to make the settings (Oliver's proposal) during the project setting (as you proposed) and not in the file.
But alas, to no avail. AT is still not available.
Actually, during the project setting phase, so far as I could see, there is no possibility to select the AT option.
Once the source and target languages have been selected, SDL automatically displays the fitting TM (from my list of TMs) and shows at the same time that AT is n/a.
It doesn't matter whether I select 'US English' or 'UK English' similarly it also doesn't make a difference whether I set up a project or proceed to setup translation as a 'single file'.
The TM window image in SDL always looks like the one I have already attached in my original mail.
So, there must be something else which is not functioning.
But why? After all it was working three weeks ago!
Looking forward to your further advice/suggestion.
Thanks and best regards