In 99,9% of cases this error occurs it's due to a damaged cookie table. To solve the problem either delete and re-create all cookie.* files below AFP application directory with Microsoft Visual FoxPro or just empty the table with the ZAP command and exclusive access to the cookie table.

lcSafety = Set("SAFETY")
SET SAFETY OFF
USE cookie IN 0 EXCLUSIVE
ZAP
USE IN cookie
SET SAFETY &lcSafety