I had this issue as well with 4.5 it seems to be caused by the new menu system in 4.5.
Delete any nodes that reference a “SKEL…” type and compile the blueprint, now go into your Editor Preferences and under the Experimental tab, turn OFF the Use new Blueprint Menu System setting.
Go back to the blueprint and it will work for you again. This seems to have been fixed in version 4.6 preview that uses the new menu by default, so you shouldn’t have any trouble when upgrading to 4.6 when it is released.
Here is the post I made back when I first encountered this with some more info: