>We received this error 'Error building key for index "...filename.cdx" tag "tagname"'
>the table is a fox2x free table, and the tag expression is a single field N 1,0.
>I looked at all other tags in the CDX, and they all seem valid (i.e. no memo, no variable length expressions...)
>I did not reproduced in debug, so I don't know the line of code, and don't know yet if it is consistent. In the form method where it happens, there is an INSERT INTO... the file named in the error, and the field used in the key is not specifically assigned a value in the insert. Could that be the cause, and if yes why?

Maybe index or table is corrupted? Don't you've error handler in place that logs error info for future review?