PHP Errors if member tries to post in any forum

As admin, forum posts (in any forum) work just fine, but soon a regular member try to post a forum topic (in normal or group forum), he get this errors displayed, all on one page:

reset() [function.reset]: Passed variable is not an array or object in C:\apache\htdocs\social\sites\all\modules\og_forum\og_forum.module on line 749.

key() [function.key]: Passed variable is not an array or object in C:\apache\htdocs\social\sites\all\modules\og_forum\og_forum.module on line 750.

array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in C:\apache\htdocs\social\sites\all\modules\og_forum\og_forum.module on line 751.