I think this started just before the 2.5 tag (r22133) but confirmed on current head (r22303). Looks like it's a server side issue as it's happening on both HTML and Python clients. See:
Screenshot
I can reproduce this with latest trunk builds on MS Windows. This a server side bug.
Bisecting:
So the problem comes from r21906.
Fixed in r22316. (char vs int)
Thanks for the bug report! (not sure how I didn't spot this earlier)
Thanks r22316 fixed it, think it might due a backport to 2.5 also.
The fix has been backported to v2.5.x in r22317.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2252