Timeout but "Remember Me?" is Checked
 

We have tested vBulletin for several weeks on a separate domain "testdomain.de" together with a drupal installation and the drupal vb module. The forum content came from an existing phorum5

To force users to stay logged in* we modified the template as described in this posting

Everything worked fine: Users could login via drupal and were logged into in vB too. A logout in drupal _or_ vB logged them out in both systems. 15 minutes of inactivity removed them from the active user list* but the stayed logged in. :)

Then we decided to convert our phorum5 to vB and go live.
After importing phorum5 data* exporting vB settings from the test site and replicating the template modifications everything seemed to be okay. Everything* except that all users get a logged out after 15 minutes of inactivity!

The cookie settings on the test side were:
Cookie path: /vbulletin/ (this is the folder of the test installation)
Cookie domain: .testdomain.de

So we tried on the live site the corresponding settings:
Cookie path: /forum/ (this is the folder name of the live site)
Cookie domain: .livedomain.de (in the user defined settings)

Did'nt work. After reading several threads here and on the german support site* we tried the tools.php script and resetted to

Cookie path: /
Cookie domain:(empty)

But users are still reporting that they are logged out after the timeout (which is currently set to 10 hours).

Is there anything else that we can check/repair/configure?