Water in fuel tank.

Probably a blocked breather or drain pipe or the stub where they attach to the bottom of the tank is blocked. This lets water back up in the area under the filler cap. What you need to do is lift the tank and pop off the two breather pipes underneath (it's ok, fuel won't come out). You'll then need a small drill bit to clear the stub(s). Also make sure the two rubber pipes that exit under the bike are clear and not kinked. 

Had this on mine as well ijust got a piece of wire and pushed it down the breather pipes then checked the rubber drain pipes off the tank the same way.