Hi @a.rozgonyi96 ,
I wanted to let you know that I’ve tested your original problem with the branch that David created to fix this problem (PR 7345) and it worked well, no errors! You can install this branch with
pip install git+https://github.com/PennyLaneAI/pennylane.git@fix-class-proc-trainabilirt
It will be soon merged into Master so once it’s in you can use the master branch instead.
I’ve made a minimal working example for the problem you encountered with ApproxTimeEvolution. I’ll add it to Forum topic 8118 which is more related to this problem.