Please post the model number of the Forward Path Attenuator. Many of these are not suitable for use with the "enhanced" upstream of DOCSIS 3.1 devices.

... FPA9-54 ...

If I'm understanding Belden/PPC's numbering correctly their FPA9-54 is a 9 db attenuator that passes frequencies below 54 MHz and attenuates above that. The problem is that 54 MHz is well into the mid-split range, and the device will be attenuating both upstream and downstream signals. We want a device that only attenuates the downstream, the "forward path". 

Mouser Electronics sells these: https://www.mouser.com/ProductDetail/Belden-Wire-Cable/FPA10-85?qs=7/6SraaimPSIHrT2p7ElHw%3D%3D. The specs are a better fit for mid-split signals like the ones Comcast/Xfinity's uses, but Mouser lists this device as "Non-Stocked" so goodness knows how long it would take them to get one to you. 

I should have asked before:

1. Do you have an amplifier in the coax line? If so, try bypassing it to lower the downstream power levels. 
2. Do you know how long it has been since the error counts were reset? It's much easier to determine whether they are a problem or not if we know how quickly they are accumulating. 
3. What is your modem's model number?

And in any event, if it's worth it to you, for $100 Comcast/Xfinity will (or should) come out and correct the upstream and downstream signal levels and clear the cause of the error counts. Or, at least, that's the theory . . . 

One hour since the reset of counters ...

Thank you! That's too many uncorectables for just one hour. Much as it is likely to be painful (financially and otherwise), I'd suggest having a Comcast premise tech out to take a look. There seem to be multiple problems (high error counts, high downstream power, upstream OFDMA channels not locked) and if there's a "silver bullet" that would fix them all, I do not know what it might be. 

Note that the first message you posted this morning is still not visible in this thread (you might see it with a "Private" flag), probably because it contains your modem's serial number and MAC address. Comcast/Xfinity considers these "personal information" and won't let them post. They may or may not edit the post to removed these, or you could re-post after removing those items. I only know about the post because the subcontractor Comcast/Xfinity uses for this forum made the bizarre decision to send emails to subscribers that included the "forbidden" info, even as they suppress it in the Forum. 

Aha, I see they have posted an edited version of that message.