error installing os-net-snmp

[bdario]

Hello,
opnsense 19.1.4
I would like to put the firewall under monitoring (Nagios core) via SNMP
I try to install the plugin os-net-snmp but I get an error:

***GOT REQUEST TO INSTALL: os-net-snmp***
Updating OPNsense repository catalogue...
OPNsense repository is up to date.
All repositories are up to date.
Checking integrity... done (0 conflicting)
The following 2 package(s) will be affected (of 0 checked):

New packages to be INSTALLED:
   os-net-snmp: 1.3
   net-snmp: 5.7.3_19

Number of packages to be installed: 2

The process will require 12 MiB more space.
[1/2] Installing net-snmp-5.7.3_19...
[1/2] Extracting net-snmp-5.7.3_19: ......
pkg: Fail to create temporary file: /usr/local/man/man3/.snmp_sess_timeout.3.gz.vy7xpmoG9X6H:Input/output error
[1/2] Extracting net-snmp-5.7.3_19... done
Checking integrity... done (0 conflicting)
Nothing to do.
***DONE***

Please can some guru explain me where I'm wrong?
Best regards
Dario