So first: with the images from
Fairphone_FP3_8901.2.A.0105.20191217_12171325_user_release-keys_dump by k4y0z it finally worked to flash the stock image by image. Thanks a lot!
Unfortunately also the second try of flashing
lineage-16.0-20191017-UNOFFICIAL-treble_arm64_bvN.img (I tried it with the LOS 16 now) wan’t sucessfull: LOS and TWRP won’t boot again. But now I know at least how to go back to stock I will post my problem in the GSI thread again, as it is not about TWRP.