With respect to all, I appeal to posters to take account of the knowledge level of the OP, in all cases. This is often readable between the lines.
Or more simply put, if an OP was able to understand the spats and tiny differences of opinion, they would hardly be asking simple questions here.
Henning, it is totally normal for previously hidden issues to pop up as treatment progresses. Desk Reflections, Flutter Echo, or Slap echo, are all hidden amongst thousands of other reflections in an untreated room. As you add treatment they come out of hiding.
Let me be very assuring here.
Comb filtering due to a desk reflection is quite audible. Play pink noise and physically angle your desk while listening. You will hear the whoosh, like a Phaser pedal but not sweeping. It is well worth angling the desk or and moving speakers to lose this problem. EDIT The combing and it's disappearance when the reflection is fixed are both easily visible on the FR graph. Again Desktop Reflection Revealed
However nicely worded I cannot see any purpose to stating that the cause is inaudible but the result is. This is exactly the inappropriate use of knowledge I referred to earlier.
Slats are nice, they look good and keep drunk drummers from disappearing into the fibre behind. They do not make a large or obvious change to the listening experience unless you have a very large surface area of them.
The biggest problems in small rooms normally start from the bottom up. So Bass Traps in corners is a sure thing goto always. Be aware there are corners everywhere, overhead, ceiling to wall corners etc.
If you do not have space available for LF treatment, the thin and but effective VPR is the way to go.