Well I am having the same problem but I suspect it is a browser issue.
I am logged into vB
3.5. Then hit my Admin link in PP 5.21 and PP asks me to log in. I do again and back to my gallery. Firefox, Netscape and Explorer all do that on my Mac. On my PC I can log into vB
and then click on my PP Admin link.
I have a second vB
board 3.5 with PP 5.21 and it works fine.
I've cleared cache and cookies. Anything else?