Have you got two LDAP plugins enabled, e.g. the core LDAP Server and the third party LDAP Server (Sync Plus), under Site administration → Plugins → Authentication → Manage authentication such as:
If the first LDAP plugin was configured correctly and authenticating users but the second not configured correctly then incorrect credentials would fall through to this second plugin resulting in the issue described.
(For next time: There's a specific Moodle forum for authentication issues).