Update from Jottacloud support:
Hi Simon,
We have experienced some issues with one of our backend dfs’s with upload availability so it’s possible that you are running into these issues. The team is working on fixing that problem.It’s could also possible that you are just timing out since uploads are going to take quite a while with your bandwidth limitations. You are currently storing 44TB data.
Guess we have to wait until they fixed their DFS.