[Andreas]

The Forum Manager uses the Forum Datamanager to handle data from table forum and make sure that everything is correct.
But the datamanager does not know anyting about 'forumhomeicon'.
Therefor you get this error.

Although you can edit class_dm_form.php, the preferred way is doing this via a plugin:

hookname = forumdata_start

PHP Code:

$this->validfields['forumhomeicon'] = array(TYPE_STR, REQ_NO);