Problem

The error

pfsense /services_snmp.php: XML error: Undeclared entity error at line 166 in /conf/config.xml 

Is shown under Status \ system logs after saving the SNMP configuration in PFSense.

Cause

The SNMP xml file doesn’t support UTF8 characters such as á, é,í, ó and ú.

Solution

Use standard alfa-numeric characters.


I hope that helps