There is an odd gotcha or two in the mesh Upload process. One is from cloud storage programs. If one has Skydrive or a similar software installed it can be nearly impossible to upload mesh without crashing. Some times deleting the SLM file in the folder with the Collada DAE file can solve the problem.
Another problem is the file selection window stalls the viewer. If you take too long to open a file for upload, you may be disconnected. Since the file dialog stalls the viewer, the server sees you as disconnected. At some point the server times out the connection for no activity and disconnects you.
Mesh upload does NOT have the time problem other types of asset upload have. So, you may get disconnected uploading a texture but not a mesh. On the other hand your Skydrive will crash you on a mesh upload but, may be not with a texture.
It can be confusing trying to figure out what is happening. You have to look in the viewer log to figure out which crash is biting you.