Just for additional information in case it helps someone get to the bottom of this:
I run a small private Exile server and after the 1.90 update last night I (and the others who play on the server) were no longer able to connect - the dreaded "Session Lost" error.
I tried deleting the cache.ch file: the server regenerated a new one but we kept getting the error.
I then tried disabling signature verification: that worked.