problem to register error -not possible to fomplate

[avitor]

this error i have when i try to register to forums
tha page of register is shows ok
when press on complate register is shows this error

Fatal error: Cannot instantiate non-existent class: simplexmlelement in /home/content/a/v/i/aviturpiya/html/muleforums/includes/functions_vbsfs.php on line 45

i disable the plagin from vbulletin optins=and complate register works ok
but ? is not shows home page anymore

and gote this eroor

Fatal error: Call to undefined function: print_portal_output() in /home/content/a/v/i/aviturpiya/html/index.php on line 47

and not possible to log to home page

i try whet to put this - define('DISABLE_HOOKS', true); in place you wright me

and not helps

--------------- Added [DATE]1210365140[/DATE] at [TIME]1210365140[/TIME] ---------------

please i nead help
my site is close cose not posibble register