When installing PowerPivot in a multi-WFE scenario, you may need to manually register the Microsoft.AnalysisServices.ChannelTransport.dll on WFEs that do not have PowerPivot installed.
To do this, run a command prompt with an Administrator token. Navigate to (and this path may change in the future, but hopefully not):
Once in that directory, run:
Reanalyze the problem in Health Monitor and the issue will disappear.