I was able to reproduce the error, and narrowed it down to this: When Ubuntu is upgraded to 10.04, php is not reading $_COOKIE variables in the $_REQUEST arrays. So, I believe this is a bug in Ubuntu, Apache, or PHP. I'll post the issue in some forums to see if others confirm the bug.
I only tested it as in your setup in a local environment, and found a work around by changing a couple of lines in one file. If you replace /admin/passcheck.php with the attached version, it should work.
I will make a change in the next release that will not rely on the cookies at all, so even if this is not corrected in Ubuntu installations, it won't affect SOS Webages.
Let us know if this works for you.