>Someone is opening the table (assuming it's a table) in exclusive mode. If that shouldn't be occurring at all, then SET EXCLUSIVE OFF or USE table SHARED in the part of your code that does not explicitly opens the table in non-exclusive mode.
>
>Note that DELETE and RENAME also require exclusive access. The same error will trigger if you attempt one of these on an opened file (regardless of exclusiveness mode).

In addition:
PACK, ALTER TABLE, REINDEX structural index also require exclusive access. Reindexing very old style .ndx did not, hazy about non-structural cdx...