[TYPO3] Error! Reason: ID was outside the domain

[Robert Markula]

Hi folks!
Beside the main pagetree (baseURL = http://www.domain.com/) I have a 
subdomain with its own pagetree (baseURL = http://subdomain.domain.com/).

Whenever I'm on a subpage of the subdomain (e.g. 
http://subdomain.domain.com/level2/), clear the FE cache and reload the 
current page, I get this error:

Error!
Reason: ID was outside the domain

When I go to the main subdomain page (http://subdomain.domain.com/) and 
go to the same subpage again, the subpage is displayed without any problems.

Any ideas?

Cheers,
Ro