Upgraded to PHP5, but i still got a fatal error.
But then I found out, that i had to deactivate Falbum prior to activating WP-Openid, et voilà – it worked.
When i try to reactivate Falbum, i get another error:
Fatal error: Cannot redeclare class Log in /www/htdocs/xxx/wp-content/plugins/falbum/lib/Log.php on line 38
Looks like they don’t like each other…