Well, it does seem like there was a slightly elevated level of FECs on the 20th around midday, which of course preceeds the issue you mentioned in regards to high levels of ES. But you didn't start seeing CRCs (which lead to ES) until the 21st as previously mentioned.
While this may be speculative, I thought it was something to do with the Bit Allocation Table (BAT). I have just taken another look and it seems that around the time when the FEC levels started elevating at around appear to coincide with sudden bitswaps on the upstream at around 11:40 and 12:40 on the 20th on a line which, from what I can see, does not experience a lot of bitswaps in that direction.
There is only one reason why I doubt myself and that is because if there is an issue with the BAT (that the modem gets when it syncs) then usually the SNRM is usually on a downward spiral as the average SNRM across the tones in use plummets as it struggles to find and allocate tones that have the required margin.
Hopefully someone can add to, or debunk this speculation.