EOF error in master/detail form


I've got a problem with a master/detail-form. The form uses a TTable for
the master and one for the detail. The detail table is filtered. I use
cached updates, so that I can insert data to the detail without being
forced to post the master record. I also set the index of the master to
a secondary index. Now if I try to post a new record I sometimes (not
always!) get an error message End Of File. What am I doing wrong?

Thanks for any suggestions in advance.