I think I've figured it out but do not really understand the syntax:

'required|unique:assessments,identifier,NULL,id,user_id,'.Auth::user()->id);'

I am not sure why I need NULL and id in there? It produces:

"select count(*) as aggregate from `assessments` where `identifier` = ? and `user_id` = ?"

From the Laravel docs on validation rules:

unique:table,column,except,idColumn

So, in your example, the third argument, NULL, is the except bit. It's null because you're not trying to ignore a given ID. The fourth argument, if I read the docs right, is the column containing the id field... probably just id. The fifth and sixth arguments are what populate your where clause, in your example, WHERE user_id = Auth::user()->id

Again, from the docs for the third argument,

"Forcing A Unique Rule To Ignore A Given ID"

'email' => 'unique:users,email_address,10'

This is NOT working:

'short' => 'required|unique:kunden,id,6'

also tried: 'short' => 'required|unique:kunden,6'

since i cant find the information if i need to specify the field the 6 is related to or not.

I edit data with ID 6 and it still tells me i cant since short is existing already.