We're running 2 Windows server deployments on FMS v14, mainly for web direct and do occasionally have to restart the worker due to web site failure. We've not as yet upgraded to v15 so nothing to go on as yet. Also, at this stage we've no diagnosis as at the time it is just a question of getting it running again, which the restart resolves. IIS restart alone does not solve the problem.
Not too much help to you other than to confirm the problem remains at FMS v14.