Beginning with Visual DataFlex 17 and now even more so with DataFlex 18 functionality of overlap fields in the sql connectivity kit has been broken and will probably not be fixed by DAC.
In any case i suggest not using overlap fields but if you have a legacy system and you are suddenly getting the following error
Invalid character value for cast specification
prepare to remove all your overlap fields.
In VDF 17 overlap fields consisting of DATE and NUMERIC fields a were broken
In VDF18 overlap fields consisting of multiple NUMERIC fields were broken
The issue in VDF17 was reported to DAC and reproduced but as far as we know never fixed. The issue in DataFlex 18 has been reported