Hi,
That error seems to be raised from the Npgsql driver. I'm not running the very latest version so you could try obtaining the latest npgsql.dll and replace the one that shipped with mojoPortal and see if it is a bug that has been fixed.
There is also more information about Npgsql here:
http://npgsql.projects.postgresql.org/
and on this page:
http://npgsql.projects.postgresql.org/docs/manual/UserManual.htm
it shows connection string settings, you might try adding Encoding=unicode to your connection string
The scripts that ship with mojoPortal make the database use unicode not UTF8, I would think unicode would support Russian characters so why not try creating a db with unicode instead?
Hope it helps,
Joe