postfix broken

This is duplicate of node/59548 but once did not get answers there…

After the 11th of November (possibly after an update) I stopped receiving emails.

LAst Friday I reinstalled postfix (several times) but I a not being able to fix it.
When I try to send email, I get MAILER-DAEMON (user lookup error).

I am clueless about what to do (after an entire day of efforts).
Any idea of how can I debug and fix this?

Thanks in advance

Webmin version 1.900
Virtualmin version 6.04
Debian Linux 8