I give up for now with bn-mining, it's a waste of my time. 4 Windows and 1 VM Ubuntu machine constantly spilling with "EXCEPTION:12bignum_error
CBigNum:operator/ : BN_div failed" or St12 , St13 Exceptions, sometimes they work for some time with blockchain in sync but stop mining, it's easy to check, when processes are using less than 20 % of CPU and less than 800MB of ram.
Ignore the exception, it only means the bidtracker was unable to retrieve all data. The falling out of sync issue is likely connection related, make sure you aren't just connected to one peer or multiple peers on outgoing connections only.
I can't ignore this exceptions because it ends with bn-mining stoping or wallet crashing even on ubuntu. This is only about individual pcs but on rather good connection, yeah firewalls, routers, etc. could be connected with this problems, but I cannot avoid that.
Remote BNs on VPS are stable as never (I don't bn-mine on it), but only on latest master branch wallet, previous test versions gave me a headache, so I will not try latest test version on it.
So to sum it up, now I am testing latest version (master or rc) on individual pcs, when it's stable enough then it goes to BNs on VPS, if not then I will stay with previous version.