There must be more detailed error information even at default log level.
I never hear about such problem, so this should be something in your network 
Check for SSL encrypted transfer in this thread http://communities.vmware.com/thread/333786?start=0&tstart=0 this could help you.
If you want to schedule your conversions try to change 'Maximum Concurent tasks' to 1 and tasks will be executed one by one.
HTH