Check the last tab of the GUI is it preferences?  If so check the meantone.  The magic figure has to be 440.  It is possible you knocked or changed something here.  Also note that yo7 have other meantones for different periods in time.  440 relates to A that an orchestra tunes itself to. In Bach’s time this was slightly different to what the modern orchestra uses now.

Also remember that if you wrote your score in score pitch rather than actual pitch i have found that when exported from MuseScore using the xml file you can get issues with instruments that are in a different key to the rest of the orchestra ie strings are in c major then for come woodwinds and brass have Bb as the same as not that a viola would play as a C.  I am assuming you knew this?  I got caught out as a kid because i didn't know not all instruments are the same pitch as a string instrument.  We hadnt done transposition.

Thanks for your suggestions. I checked Aria Player's tuning and it's European A442. And it doesn't seem to be a transposition problem.

Anyhow, I tried different things since my first post (at one point, mysteriously, the pitches were one half tone too high...). Finally I upgraded to Cubase 10.5.2 and downloaded the last version of Aria Player and now everything's OK. I just hope this will last...