Hi Verena,
Just to be sure: when you ran basis_tools convert, did you specify the bandwidth of your data (2000 Hz) or of the basis set (4000 Hz)? I believe the latter is correct in this context… ie:
basis_tools convert RawBasis_for_PRESSPhilips_TE_35_BW_4000_NPts_2048 --bandwidth 4000 --fieldstrength 3.0 fsl_basis_dir
Alex.