Thanks. I tried it once on a machine that had it installed from last year - hence should have been licensed already but somehow wasn't - and got the error. Then I tried downloading and installing it fresh - the 2021 version of the app - on another Mac and got the same error. Same half-window that can't be expanded and same space for the license code that allows input of the code but with no possibility to hit return and actually enter it. So nothing to do but hit 'cancel' at which point the app quits.
