It looks as if it is something to do with the data format. A lot of programs can read FP files now (even Astrometrica) but they assume that the data is scaled in the same way as an integer format file, i.e. a range of 0..65535 and they just truncate to the nearest integer. FP format FITS files can have some wacky scalings. I’ve seen some where the entire image dynamic range is in the range of 0.0 to 1.0 and that clearly doesn’t work very well if you just truncate. From looking at the header of your calibrated file that does not look to be the case here.
The headers look OK with nothing obvious. Do you get any hint in the error message from VPHOT or does it just silently die? As a workaround can you get ASTAP to save your calibrated files in integer format? I would have thought that there would be some option to do this.