With layout set to gr (locally/remote), there are warning errors (and keys don't pass through) about xpra HOT 2 OPEN

That's not easy unfortunately!
This commit d49e84f (6 years ago) added a workaround for Gtk failing to convert the event key string into something we can use without throwing an exception. It remains in use to this day...

We could generate our own map, or perhaps change the server code to do the lookup based on Greek_UPSILON alone, without using the string representation.
Either way, I'm not sure how to convert Greek_UPSILON to a string automatically.

from xpra.

It might have to do something with how the keys end up on the server - and somehow its keyboard language is changed

Some Alt, Shift shortcuts don't pass cleanly, e.g. Alt+` to cycle windows.
That usually leaves Alt activated on the remote side, even if I have released the keys locally.
"Somehow then", it ends up triggering a layout change on the remote - causing the issue that local en/gr keys cannot be then "sent over" to the server.

But if it's that complex, let's leave it. I'd be nice if yet-again modifier keys would be passed on / reflected cleanly on the remote.
However, "even that is broken" since, activating Alt+Shift as a language-changer, it blocks any Ctrl+Alt+Shift shortcut
(and similarly, using Ctrl to "find mouse" also does "something else")

from xpra.