Found it.
If you want to fix it yourself, edit the admin template
forum_edit and change the following code:
Rich (BB code):
<xen:option value="{$prefixId}" selected="in_array({$prefixId}, {$nodePrefixes})">{$title}</xen:option>
to this
Rich (BB code):
<xen:option name="available_prefixes[]" value="{$prefixId}" selected="in_array({$prefixId}, {$nodePrefixes})">{$title}</xen:option>