Logs:

SQL: SELECT c_password FROM sogo WHERE (c_uid = 'example@example.com') OR (mail = 'example@example.com');
2019-01-22 18:14:10.892 sogod[4311:4311] <MySQL4Channel[0x0x559f4d43b730] connection=0x0x559f4d6d1628> query has results, entering fetch-mode.
2019-01-22 18:14:10.892 sogod[4311:4311] <MySQL4Channel[0x0x559f4d43b730] connection=0x0x559f4d6d1628> SQL: UPDATE sogo SET c_password = '12345' WHERE c_uid = 'example@example.com';

Update query fails because our c_uid is a login (example). If users log in via his login (example) instead of his mail (example@example.com) query has success, but if they log in via mail, query fails.

+-----------------+--------------------+------------+--------------------+--------------------+
| c_uid | c_name | c_password | c_cn | mail |
+-----------------+--------------------+------------+--------------------+--------------------+
| example | example@example.com| 12345 | example@example.com| example@example.com|
+-----------------+--------------------+------------+--------------------+--------------------+

The fix could be something like: UPDATE sogo SET c_password = '12345' WHERE (c_uid = 'example@example.com') OR (mail = 'example@example.com');