The only way I can upload to the gallery is to log in there 1st. Then I can upload to the gallery plus anything else I want in the forum.
If I logged into the forum 1st I have to log out, clear my cookies & then log into the gallery.
I don't know why that is, but it is always like that for me.
If you just remember to always go to the gallery 1st & log in then you want have a problem anymore.