[Error Message] Error when creating new forum in new category

[nuva]

I am creating new forums in my new myBB installation, but whenever I add a new forum to a newly created category I get this error:

Warning [2] Invalid argument supplied for foreach() - Line: 2657 - File: admin/modules/forum/management.php PHP 5.5.9-1ubuntu4.5 (Linux)
File Line Function
/admin/modules/forum/management.php 2657 errorHandler->error
/admin/modules/forum/management.php 2273 build_admincp_forums_list
/admin/index.php 614 require

After this no categories or forums are displayed on the home page anymore and I can't do anything in the admin on the 'Forum Management' page.

Is this a common problem or have I forgotten to set any settings?

Thanks for the help.

[dragonexpert]

ACP->Tools->Cache Manager->Rebuilt Forum cache.