I have more news on this strange behavior...
I killed all the cookies from yesterday out of the IE folder "Temporary Internet Files" and it worked with foo.com and sub.foo.com!!! I logged in the same user on both sites as well as different users on both sites and I couldn't recreate the error. After couple hours I retried to log in.... and the same error as before occurs: I just can't log into sub.foo.com but I can log into foo.com!!!! STRANGE...
Then I logged out of both sites and evaluated the cookies. They are different.
for sub.foo.com:
ad_user_login
3,726F796264404066
sub.foo.com/
0
3190974464
30050941
3018160352
29511313
*
AND
for foo.com:
ad_browser_id
204535
foo.com/
0
3190974464
30050941
2358065712
29511278
*
ad_last_visit
1030535689
arasis.de/
0
3190974464
30050941
2053370352
29511313
*
This means that he didn't really log out with sub.foo.com and doesn't have a browser_id anymore. After I just typed in sub.foo.com/register/logout.tcl the sub.foo.com cookie looks like this:
ad_last_visit
1030536486
sub.foo.com/
0
3190974464
30050941
1429595760
29511315
*
The browser_id is still missing and I can't log into sub.foo.com at all!
Then I delete the cookies again and the old error occurs. STRANGE!
Then I close IE completely AND delete the cookies. I *can* log into sub.foo.com, great, but what's wrong? I am going crazy...
Any cookie chief out there?