Emptying the session table is ok for trouble-shooting purposes - if you do it while the site is live you'll end up bumping the users off if they are logged on.
I'm wondering your issue is the same as the one described here:
http://www.ecusb.com/index.php/topic,911.0.html - that ended up with Esvon/Vladimir changing a couple of files (the post at the bottom of the thread has the details).
The cannot login after site move scenario has been reported a few times on this board, I guess your best bet would be to contact Esvon for assistance.
hth,
Charles