Unwanted offset attenuation in FIR filters

Everything in this measurements is "unsigned", when there is no other remark. I repeated the whole measurements with everything "signed" and had comparable results (although the permutations were different). I found no combination that worked "right".

The filter input signal comes from a Tektronix AFG 3101, and should be completely DC-free. There is nothing in between the devices and instruments exept BNC cables and a "mini circuits" DC-48MHz alias filter at the DAC output.

The miniVNA (vector network analyzer) has 10dB attenuation at each in- and output to protect it - but these are calibrated out.