Dear Hauke, the conversion of intensities to amplitudes is an area with unsatisfactory solutions, and the approaches of (and the assumptions made by) the different programs differ. Low-resolution data are often anisotropic, which adds difficulties. You could try CCP4 truncate/ctruncate and GlobalPhasing's staraniso. The latter in particular should treat anisotropic diffraction more properly. Probably you get yet other answers from xtriage when fed with those amplitudes. Possibly this will add to the confusion, but I'm not sure it is worth worrying about this too much. Having said that, it seems to me like the primary and most unspoiled data are the intensities. I don't see anything suspicious in the xtriage-on-intensities output that you posted.
good luck with your difficult project! Kay