Thanks for responding.
Ye, that was the case, and i thought there might be a problem with that, so i deactivatet my phones lockscreen code and it still didnt work.
Im sorry, i forgot to update the post.
I didnt manage to find the source of the problem but i could still manage to fix it. On the following post the answer helped me out. While i could not successfully enter the generated code from the fp3 bootloader unlock page, i could still unlock my bootloader using
fastboot oem 8901_unlock and in the end everything works fine with the phone after installing /e/.