Problems with v1.5.10 - 500 Internal Service Errors

The following information is required to get an answer:
Get this information from the PHP diagnostic (in the configuration wizard).
Gallery URL (optional but very useful):
Gallery version: 1.5.10
Apache version: ?
PHP version (don't just say PHP 4, please): 5.2.8
Graphics Toolkit:
Operating system: Win Vista Ultimate
Web browser/version (if applicable): FireFox 3.0.6 / IE7

I've recently implemented a pre-launch test version of the Menalto Gallery v1.5.10 - prior to upgrading my version 1.44 gallery.

They are located as follows:

www.brigatti.co.uk/gallery2 (for my current main released version)
www.brigatti.co.uk/gallery3 (for the 1.5.10 test version)

I have been trying to test the new version out, but there does appear to 1-2 bugs impacting it - mainly focused on the log-in/log-out function, new album - all of which create a 500 Internal Server Error - when calling upon the respective command!!

I have found a quick way to resolve the login/out issue on the gallery - but wanted to raise this formally with you all first. The quick-fix appeared to be to delete the call to the URL return reference, i.e. reference to lib\content.php - where I amended the following two lines
$returnUrl = makeGalleryUrl('login.php', array('g1_return' => $return, 'cmd' => 'logout'));
$returnUrl = makeGalleryUrl('login.php', array('g1_return' => $return));

by removing $return and replacing with '' - okay I lose the return URL reference and return to the root of the gallery when I login or out, but I was able to live with this. On the site URL I've referenced, I have reverted back to the original erroneous code so you can see the effects of the reference as-is (without my correction).

The new gallery issue was also resolved, after I clicked on the link, by means of removing the return-url reference on the actual URL after the 500 server error appeared, e.g. instead of:
http://www.brigatti.co.uk/gallery3/do_command.php?return=http%3A%2F%2Fwww.brigatti.co.uk%2Fgallery3%2Fview_album.php&cmd=new-album

http://www.brigatti.co.uk/gallery3/do_command.php?return=&cmd=new-album

Can you let me know your thoughts on this.