I'm still having some problems with difficulty and results not being accepted by the pool after some time the miner is running with xmr stak, this is a recent thing never happened before.
Question: can a fixed difficulty be specified somewhere (I read of putting an address+diff in the login) or is completely managed by the pool?
Regarding both your questions:
-Today was two pool restarts, maybe this cause the error. Not sure and still didn't get any similar problems from other users.
-As for the fixed difficulty - it's fully managed by pool, so you can't make it fixed. But I'm almost sure it's not a problem. Did you check your hashes on our side while mining? I mean when xmr-stak doesn't say you "block accepted by pool" are you sure that pool really doesn't?