Compose keys don't work in LyX anymore

[John Coppens]

Hi guys...

I'm hoping your collective knowledge helps me out.

I'm not a frequent LyX user (2.3.6.1) but when I had to edit a document
with accented characters (á, é, ... and ñ's) this doesn't seem to work
anymore. As you can see, in the mail program it _does_ work. 

Also, I'm quite sure this worked before. When I type Win + ' + a, I get 'a
on the screen.

I have a suspicion that the problem comes from Qt5.15. Most of  my applications
are Gtk based and work fine. I am still running X11.

It's not a UTF-8 problem - I can still copy&paste from other texts into LyX.

I tried to redefine the Compose key onto the CapsLock - no change. Gtk works,
Qt doesn't.

Anyone know what could have happened?

John