The problem was the FS.com DAC, I got the XS+DA0001 DAC from Mikrotik and it works great.
Thanks for the suggestion, I will try that.There were problems with using the SFP+ port on the 4011 with DAC cables in the past. The issue has been fixed in RouterOS, but we still use optical connections on 4011s and have no issues.
I would change to optical and see if the FCS errors continue.
I have the same situation.Gentle nudge.
I need a new router, choice is 4011 or ER4, one has some features I need and the other has SQM. Please make my decision easier!