So, I destroyed the LUKS header of userdata, formatted and resized data in TWRP, but still no luck with encrypting LineageOS afterwards .
Did the same things again and restored Fairphone Open OS 17.06.4 from a TWRP backup to establish 2 things:
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FM Radio does work in the current Fairphone Open OS (I checked, because I last used it back in Android 5).
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@Roboe: Fairphone Open OS has once again happily encrypted my data partition, at least both the OS and TWRP want my password and seemingly decrypt the data.
I will restore my Internal Storage for now and continue using Open OS for the time being.
When I have some time again, I will try to set the phone back to the state it was delivered in and then try to install LineageOS from scratch.