i have kind of the same problem...brushes, etc. work fine on pen side, but when i flip the pen over, the eraser doesn't work *until* i actually select the eraser from the tool bar? (>.<)
i'm using Win 8, Ps-CC 2014, intuos 5 large.
I'm having the same problem as studiorobb
I'm having the same issue Win10, PS CC 2015.5.1, Intuos 5 large.
I'm surprised there isn't at least a link to how to fix this here even if it's not an Adobe bug. I'm now assuming this is very broken.
Ok, sorry about the double post but I think I "re-found" my issue.
I had disabled Windows Ink (I think because it was causing me issues somewhere else).
This fixed it for me:
Open 'Wacom Table Properties' goto the 'Mapping' tab and check the "Use Windows Ink" box.