When setting the following:

? You are here: Dashboard > Blog A > Settings > URL configuration

? Category URLs:
? Use param: cat IDE-g: http://demo3.b2evolution.net/stable/blog1.php?cat=123

When the following link is followed:

http://demo3.b2evolution.net/stable/blog1.php?cat=5

The following error occur:

Fatal error: Call to a member function get() on a non-object in /home/b2evodemo/www/stable/inc/skins/_skin.funcs.php on line 905

However, when:

? You are here: Dashboard > Blog A > Settings > URL configuration

? Category URLs:
? Use extra-path: sub-categoryE-g: http://demo3.b2evolution.net/stable/blog1.php/subcat/

Or

? Use extra-path: category pathE-g: http://demo3.b2evolution.net/stable/blog1.php/cat/subcat/

Then it works fine.