Using the minus sign (-wordxyz) together with a 'by user' produces this error. OK with no 'by user'.
I think I've seen this long time ago, so probably not related to recent updates etc. No index.
Not unique table/alias: 'm'
File: /customers/1/xyz/httpd.www/forum/Sources/Search.php
Line: 1352
Note: It appears that your database may require an upgrade. Your forum's files are currently at version SMF 2.0.17, while your database is at version 2.0.7. The above error might possibly go away if you execute the latest version of upgrade.php.