I guess stratum is NOT working as it should.
Yes, Claymore's miner is not working properly with Minergate + stratum. Claymore will have to update it.
I tried Claymore CryptoNote CPU Miner v3.2 Beta without problem. Now I am using wolf's cpuminer-multi-wolf-05-30-2014 which is faster on my PC. Now I have Last PPLNS reward 19 hours ago but I am still minning without break. I hope that Minergate doing some updates and even this will be fixed.