Addendum:
Mike#2 suggested looking at libssh, as that was upgraded, too. So it might be that it's not a bug in x2goclient per se, but in how libssh talks to ssh agents.
-Stefan
And another addendum:
Swapping out libssh.dll against a copy from an older, still working x2goclient doesn't work. When you attempt that, x2goclient won't even start.