Hi Johan,
One of two scenarios. Either the data file does not match your dictionary (which is the most likely cause) or the data file is corrupt. To check for corruption, use clarion's TpsFix.
To compare the data file to you dictionary, I would import the suspected datafile into your dictionary and compare it to the original file in your dict. Before the import, you might have to rename the original table name in your dict (so make a copy of the dict). You can also export the dict to txd and read the structures in notepad ++ or so. You might want to check out DMC....I think the DMC tool will do a compare of two files structures for you. Or TopScan both files and compare the two structures side by side.
Cheers
Rob