Niet meer kunnen inloggen na plugin update

Door een update van een plugin kan ik nu niet meer inloggen om bij het dashboard te komen. Ik krijg verschillende foutmeldingen.

o.a.
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘ass_init’ was given in /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-includes/plugin.php on line 405

Daarnaast krijg ik bij inlogpoging de foutmelding:
Warning: Cannot modify header information – headers already sent by (output started at /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-includes/plugin.php:405) in /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-login.php on line 349

Warning: Cannot modify header information – headers already sent by (output started at /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-includes/plugin.php:405) in /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-login.php on line 361

Warning: Cannot modify header information – headers already sent by (output started at /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-includes/plugin.php:405) in /home/cbeaca01/domains/cbeacademica.nl/public_html/myacademica/wp-login.php on line 363

Wat nu te doen!!