Okay found the annoying line...I guess this was supposed to be one of those "features"(!).It is just good for nothing.You know why ?

a.)You can forget to "uncheck" the default cat. when writing a new post.
b.)But you do NOT forget to "check" the cat. you want.Especially if it's a maincat, it get's selected as extra cat automatically already.

I can think of one *possible* use for this "feature"; That is IF
it causes a malfunction when the main cat is not selected also as an extra cat; and AFAIK it does not.

The worst part is; I dont have a default cat.So this is not only a usability issue and it seems it is also a "bug".

Might sound so small, but when it combines with a lot of little annoying things about 4.x it may get easily unbearable.

Anyway, if you have the same problem uncomment the below line:

..\inc\items\model\_item.funcs.php

				{ // This category was selected or if wasn't select any category but this is the default
			//		$r .= ' checked="checked"';