@Markus: I already knew all of those points and I did not mean to insult you. Nor did I intend to say that letter is better than scrlttr2. In fact it's the disclaimer in this bugtracker which requests to check wether the issue does not occur with standard classes. I just think this should be listed as a todo item. Unfortunately my skills do not enable me to solve this myself.
scrlttr2 not compatible with tagged PDF