Website doesn't seem to recognize shortcodes resulting in heavy errors

Hello there

Since I’ve installed my Membership plugin, I’ve been experiencing a lot of errors.

The plugin does not work. I’ve installed the plugin as stated, tried to solve the problems by de-activating, changing theme, etc. None worked. When browsing as admin user, I do not experience these problems, but when browsing as a visitor (not logged in) I keep getting the following errors:

Warning: preg_replace() [function.preg-replace]: Unknown modifier ‘b’ in /home/ciofo1/public_html/be/wp-includes/formatting.php on line 324

Warning: preg_replace_callback() [function.preg-replace-callback]: Unknown modifier ‘b’ in /home/ciofo1/public_html/be/wp-includes/shortcodes.php on line 151

When browsing as admin, I also see the shortcodes of the plugin ([level-ciomember] and [/level-ciomember] in my example) like text, so they don’t do anything.

Thanks in advance for solving this for me.