One side note: In 1.5.25 it first redirects you to your homepage and after one more try you can login in your admin backend.
It's definitely a bug and should be removed, otherwise I would say KUDOS to you guys, keep the good work!
Greetings from Croatia!
The problem you describe is caused by a Joomla bug introduced in Joomla 1.5.22. The issue is documented in the Joomla bug tracker in issues 23216 and 24976 (it's actually just 1 issue). There are several proposed fixes, but none have been implemented.
This one has been open for more than a year.
This one has been open for almost a year.
I agree that it should be removed. Unfortunately, I've done all I can to resolve it - the issue is within Joomla.