Hi Steven, thank you for your suggestions. Yes, I have installed MultiTerm 2017 (after I installed Studio though). And referring to the termbase, no, I haven't actively added one. However, it is not only happening in one project, it happens to all, so it seems more a general configuration problem than a problem with the project configuration.
And this is what I see when I try to add a word to the dictionary:
Thanks for your help!