Does not matter if the AF is new out of box, ports are sometimes bad.
Plus this is not a new AF and I think you said this started happening so port could have been damaged in switch or AF.
If port was damaged in switch or AF then most likely it is from ground current hence I provided the grounding posts.
I do not know how many times people tell me their grounding is grounded properly then I get them to go over their grounding system and it is not right.
Since there are tens of thousands of switches out there powering AF units just fine then there is something wrong here at this location, you need to narrow it down to fund it. By the way ground current can cause Ethernet Errors. I know you say grounding is done properly but just saying ground current can cause Ethernet errors. Could be bad wire/end, bad port in switch or AF but if it is a bad port in either chances are ground current did the damage. You're going to have to do some swapping out of stuff to find which one.
But if you swap port in switch and swap AF unit and errors persist then it is cable/ends or interference on cable, or ground current on cable.
But anyway I provided what I would do to test what the issue is;
Well simple diagnostic skills are important:
Possible causes:
Port in switch got damaged possibly from ground current?
Port in AF got damaged possibly from ground current?
Cable or Ends?
To see if it is a port in the switch simply move the AF to another port. If the problem follows the cable to the new port it is probably not the switch port.
What's left is bad or damaged port in AF or Cable.
Check all ends and clean any dirt and re insert all ends.
At this point you are down to replacing cable or swapping AF unit.
If after switching to a new switch port the problem remains and if you swap cable then your down to AF Ethernet port damage most likely from Ground Current.