Do you still have the Plink log from 14th June that you could post for comparison purposes?
Here is the Plink log from the 14th June at 0900 
See the attached animated gif for your connection.
This portrays the slight changes (deterioration) in attenuation, Hlog, QLN, SNR & therefore bitloading.
The increase in attenuation
COULD be temperature related & the decreased SNR
COULD be a result of that.
From studying various connection stats, it appears that SNR has to be around 10dB or more per tone for bitloading etc. to take place and the US attenuation cut-off point seems to be around 65dB or so.
Your U2 Line attenuation has increased from 64.6dB to 66.7dB.
That could explain the previous 'dodgy' looking Hlog graph as it was right on the edge of being able to make use of some of the tones or not.
Now that attenuation in the U2 band has increased slightly & noise levels have increased slightly (possibly temperature related and/or increased crosstalk), none of the band is now able to be used.
Resyncing the connection at a time when SNR/SNRM are at their peak may provide a temporary improvement, but as SNR/SNRM decrease during normal daily fluctuations, this would no doubt cause an increase in errors & DLM may have already determined a 'suitable' balance between stability & speed from its constant monitoring.
If there are no physical line issues such as HR faults etc., you may either be permanently stuck with lower sync speeds or as per my connection, you may see a slight improvement during winter.
As my line's attenuation is slightly worse than yours, I can't ever use the U2 band & I see the seasonal reduced bitloading effect etc. in the D2 band.
