After more testing, a work around I found was to remove the GUILayout begin and end area, which worked, luckily I was able to recreate the buttons without it. Although if I did need to use them I'd be stuck!
On to my next problem, without the GUI Layout in there, I now get Stack error 1. This happens the second time I go to my maps page. I think the problem could be related to an FSM that creates markers on the map, as the FSM gets stuck on a create2Dmarker action the second time around.
I tried to get around this by testing if the markers were already created with an object compare, but strangely the object variables don't seem to show they contain anything, even though I use them later to change the texture of the markers.
So I'm not sure if I'm doing something wrong, but there seems to be a few bugs in Online Maps? :/