Atari 1010 tape drive BOOT ERROR

Hi,

I got a problem with one of my 1010 tape drives.
First of all the drive looks ok. The mechanics are fine, the belts are clean and tight and the counter works.
I already cleaned the head and all connectors and contacts. Rewinding and fast-forwarding are working.

But nevertheless I always get the error message "BOOT ERROR", when I try to boot from commercial, original game tapes.

I put in the game cassette, press PLAY, hold down OPTION and START and turn on my Atari 800XL.
The drive starts and I hear the (correct) data sound from my tv speaker.
After 1-3 blocks I get the "BOOT ERROR" message on my screen and the 800XL is starting the self test.

I already tried to adjust the tape head position with a little screw driver.
When I turn the screw to the left, the data sound sounds really bad and when I turn the screw to the right it also gets worse.
So I guess the head is already in a good position.

Of course, I tried different commercial tapes. When I write a little Basic program an 'csave' it, saving works but I can't reload it from tape.

Think it is an adjusting issue or the head is kaput ?