Is your harvested muscle tight at all times? Is there any visible bulge when you tense the muscle or flex the knee? Can you sense that the harvested muscle (presumable Semitendinosus) is smaller than in the other leg?

The lump you mentioned could result from muscle adhesion.

I suspect my condition to be a bad case of that described in this paper:

Failed regrowth of the harvested semitendinosus tendon: a rare complication of tendon harvest after anterior cruciate ligament reconstruction.

My advice to you is to find an excellent orthopaedist and get Wikipedia reference-linkMRI on the thigh as soon as possible.

I had an ultrasould sonography which did not reveal anything. Just waste of time for me.

For the last months I have been focusing on stability exercises, because the knee will be unstable with so weak hamstring. The stability has got a little better, but not good enough.