Update this time worked flawless using the update app - unlike all other MM updates so far on my device. This time I didn’t do a backup (using TRWP w/o installation (see my post regarding 17.08.1).
So either my issue was fixed with the 17.08.1, Fairphone has solved a problem with the recognition of system partition changes with this update or @retsifp s speculation - that TWRP somehow modifies the system partition - was right.
Anyway - I’m happy with the real quick BlueBorne fix for my FP2!