Thank you for your feedback.
I have just checked our Rigs, all seem ok, But I have seen this behavior on the last days, (MPH is under attack?) on some coins- maybe this the coz?

The solution that we found was to simply add new ".WORKERNAME" and point the miner to this new worker.
So just do the following and tells us if the problem gone:
Change this :
-u USERNAME.OldWorkerName
with your NewWorkerName
-u USERNAME.NewWorkerName

Best Regards,
zFastminer team.