I have updated TWRP to 188.8.131.52.
After that I have tried to install pa_gapps which is for the version 5.1 so I got some errors while installing.
I had a closer look at the package I previously downloaded.
I have noticed that I have downloaded the wrong version gapps-lpmr1 instead of gapps-lp.
I was then able to start the tablet without any errors.
Thanks a lot,