After upgrading from 6.3 to 6.5 and switching from NT to Linux my forum shows as "There are 0 registered and 0 anonymous users online." all the time...

It looks like you need to refresh the page at the same second that others to get any number there at all...

I noticed that in my admin part had a blank field "Who's Online Timeframe:" that i changed to 15 minutes.

Then everything seems to work more or less like before, does this all sound logic?

Yes,it does.
If the time frame is set to 0, then there wil be 0 poeple online at any given time.

ok, thanks for confirming that, thought my community over night for a second

I believe it's cached too - so the count inside the forums isn't up to the minute, and doesn't include invisible people.