Yes, I've been seeing this as soon as I increase the download and upload threads.
But it has been occuring even when I have something like 3-4 threads (upload and download combined).
Currently is seems stable enough at the latest 749 build.
No errors in the last 8 hours, but I was also taking it very slow:
1 download thread, 1 upload thread (throttled at 10 mbps).
Prefetcher disabled.
Currently trying 2 upload threads throttled at 20 mbps, hopefully it will remain stable.
I have only been using the service for little over a week (using the latest link from the main download page, 463?), but in the beginning when I was uploading close to 1 TB I didn't run into any problems at all as I recall. Maybe the occasional "rate limit exceeded" but it all went pretty smoothly.
I ran much more threads back then too.