I've experienced it myself - I'm logged in for forum (I can post), and logged out for the top menu bar. :) Weird indeed. And only after some time passes. What's more weird it doesn't happen to me in GOG building ( :D:D ) on osx, only on my private ubuntu. :)

I've added it to bugs list recently. We have few ideas where could this bug crawl from.
I've added this topic to the bug description, so if you have any valuable input - please post it.

Please check if they have different protocols in address bar - https and http - or are they the same? + when you refresh it stays the same in each tab?

I can confirm that account dropdown in header and cart are not appearing on forum pages.
We're very sorry for that, it will be fixed 23 November around 10:00 UTC.

To see the account tab and working cart (or just log in) please go to main main page of . You can also <span class="bold">go directly to account</span> via link: [url=http://www.gog.com/account]http://www.gog.com/account

If you wanted to use users tooltip on forums:

To access chat with user that you did conversate with before, go here: http://www.gog.com/account/chat
To open a new conversation you can either go to this link:
https://www.gog.com/u/USERNAME/chat
(replace "USERNAME" with actual user name)
Or go to
http://www.gog.com/account/friends
Then click "INVITE NEW FRIEND" and search for specific user - then chat or invite him.

To go to users wishlist (if public, otherwise you'll get 404): https://www.gog.com/u/USERNAME/wishlist
(replace "USERNAME" with actual user name)

I've edited my answers, but I'll put info here too - for the time the bug appears (until 23th of November, 10:00 UTC):

To see the account tab and working cart please go to main main page of www.gog.com. You can also go directly to account via link: http://www.gog.com/account

To access chat with user that you did conversate with before, go here: http://www.gog.com/account/chat
To open a new conversation you can either go to this link:
https://www.gog.com/u/USERNAME/chat
(replace "USERNAME" with actual user name)
Or go to
http://www.gog.com/account/friends
Then click "INVITE NEW FRIEND" and search for specific user - then chat or invite him.

To go to users wishlist (if public, otherwise you'll get 404): https://www.gog.com/u/USERNAME/wishlist
(replace "USERNAME" with actual user name)

And - at the same time I'm the person responsible for screwing this one up... :(

Emachine9643:
Yes, we know about you and some other users having this problem (chat/wishlist link gives 404 after changing nickname). See this topic for detailed description. My collegues will fix it hopefully sooner than later.

Missing account button and user tooltip issues are fixed.

I meant missing entirely of course - there wasn't neither account nor sign up/log in. (and it was totally not related)
Please tell me - is point 3 true (let's say it's all http, or all https - no protocol changing):
1. Go directly to forum - there's sign up/log in, no account button
2. Go to GOG.com main page - account button appears
3. Go back to forum - account button is there

Ahhh! So that's another consequence of this bug. System thinks that you are logged out. For now you can go to main page (even in new tab) and go back to forum (or just refresh it).

I've tracked down what's happening - direct cause of not being logged in - but we don't know cause of the cause yet. ;) So - we'll fix it, but I can't say when exactly.

Issue with account menu missing on forum pages is fixed. For more info see this post:

http://www.gog.com/forum/general/the_what_did_just_break_thread/post1099