My apologies for resurrecting this post, however I recently had the same issue with UFPS. The above post doesn't say which file that was in. So what I'm bringing to this table is which file its present in for UFPS.
It can be found in Photon Unity Networking/Plugins/PhotonNetwork/PhotonClasses.cs
They use a stripped down version of PUN for their MP component, but I strongly suspect unless Photon removed it themselves the same issue is still present, and will happen again anytime new packages that referance a HelpURL are imported.
And also I can confirm what @aron_granberg is saying is 100% , removing that, fixes the issue. Keep this in mind as well for any older packages that use '@HelpURL' as well that will cause a similar issue. Just drop the '@' sign
I'm happy to link to my UFPS MP bug report if anyone requests. But as this is the A* forum I'll not link to another product unless requested to do so.