password plugin on roundcube problem...

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I Have roundcubemail v. 0.8.6
Whenever i try to update users password (password plugin) roundcube send me an error.

In log file I see this:

Sep 11 10:45:16 mail roundcube: MDB2 Error: not found (-4): _doQuery: [Error message: Could not execute statement] [Last executed query: SELECT update_passwd('$xxxxxxxxxxxxx/', 'xxx@xxxx.com')] [Native code: 1305] [Native message: FUNCTION roundcubemail.update_passwd does not exist]

How can i try to solve this?

Thanks...