Hi Hauke, so your refinement results in P1 seem to say it's untwinned. I don't see (but I may be overlooking something?) the evidence, from what you write, that it could not be P21 or P2, because any P2 or P21 structure can be refined in P1 (but of course not the other way round). In other words, I'd expect that two molecules out of the four of your P1 solution can be refined in P2 or P21 to the same (or rather better) Rwork/Rfree. That would prove P2 or P21.
If there is really no 2-fold NCS solution in P2/P21 then it is P1 with 4-fold NCS. Concerning your last question whether XDS or pointless could have mistaken a two-fold NCS along a crystallographic axis as a crystallographic symmetry element, I'd need to see the pointless table after "Analysing rotational symmetry in lattice group ...", and the relevant table in CORRECT.LP where it tests some spacegroups. best, Kay
