I have the same problem but it happens randomly.
Today I could not log in GMarks again even after being logged in
GMail. So I tried my Google account instead.
When I opened the
google.com page the German version came
automatically (I was connecting from Germany). I was logged in.
Then I clicked the link "Google in English" and I realised I was not
logged in. After logging in the english version GMarks was working
again.
It seems the cookies are different depending on where you are.