The application tried to open a Btrieve v5.x file that was previously accessed in Accelerated mode by Btrieve v5.x and never successfully closed.
Another cause is that your application tried to open a v5.x format Btrieve file using a v5.x Btrieve engine, however that same file was previously accessed by a Btrieve v6.x engine, which failed to close the file successfully due to an unrecoverable error.