Just had a strange problem, but turned out to be a simple solution...
I turned off the MDWS update on my Win7 PC (HG612 modem_stats), and waited 10 minutes. Result: MDWS reported my line to be down, and kept it tagged as "HG612 modem_stats").
Then I added the MDWS configuration to my RPi (which has been running locally for a while now), and turn on the uploading. Result: MDWS continued to report my line to be down. However, MDWS did immediately switch to reporting that I was now using the RPi version for the upload. It was clearly getting the reports, but doing nothing with them!
No change from restarting DSLstats either.
Then I realised DSLstats was logging (locally) with the wrong time. It turns out my RPi was configured to the UTC timezone. As soon as I changed it to "GB-Eire", the uploads went through as normal!