So sorry for the late reply.
The issue is that the current version gets “stuck” uploading data. We haven’t been able to reproduce this ourselves, and as a result haven’t been able to isolate and fix the problem.
A possible solution is to have your players clear their caches (Info->Diagnostics->Clear cache). The bad news about this is that any progress that hasn’t been saved to the server will be reset.
We are working hard on a new version that has a completely rewritten networking layer, and it will be available in the fall.