tnx @snevas. I put the MD5 file on my sd-card, but when I put it back in my phone and tried to install again I couldn’t find the MD5. I still tried to flash again, but again I couldn’t flash.
I didn’t transfer the file from TWRP with MTP. I don’t know how to do that, although I tried to find out. Maybe that’s the reason that I can’t find the MD5 file on my sd-card in TWRP? Although, that would be strange, since I can see the other zip files in TWRP while I I didn’t use MTP to put those on my sd-card…
I hope anyone can help me out, that would be great 
Edit: Could this be the reason: ‘Android can choose the files it presents to you, and hide system files so you can’t see or modify them. If you attempt to delete or edit a file that can’t be modified, the device will refuse the request and you’ll see an error message.’? (https://www.howtogeek.com/192732/android-usb-connections-explained-mtp-ptp-and-usb-mass-storage/)
Edit 2: 27072017 - I tried to transfer the md5 file with adb push, but it didn’t succeed. I constantly get the following answer:
-bash: ./adb: No such file or directory
the last command I used was:
./adb push C:\lineage-14.1-20170708-UNOFFICIAL-FP2.zip.md5sum /sdcard1
I put the file directly on the Mac HD…
Edit 3: pfff… sorry guys, ofcourse the file was on my sd-card of my phone the whole time (I saw I could check it in TWRP via ‘Advanced’). Transferring via TWRP or adb wasn’t necessary. But well, I still got the same problem, which I wrote down in my previous post.
If anyone can help me out… tnx!
Edit 4: Problem solved!! 
I solved it in the end by NOT queueing the modem zip and the lineage build and then flash, but by flashing them one by one. So, in my case step 3 and 4 of ‘When coming form any other OS’ (LineageOS 14.1 with OTA updates for FP2) should be done differently as is described there.