Intermittent SNMPERR_BAD_SENDTO (SNMP error # -12)

Hi, i'm monitoring two firewalls with different types of sensors (port bandwidth, disk..). The problem is the SNMPERR_BAD_SENDTO (SNMP error # -12) error that appears intermittently. As you can see in the next log entries the sensors go up and down and viceversa:

02/09/2015 09:44:07	(012) Mgmt.150 Traffic	Up	
90 kbit/s

02/09/2015 09:44:06	(013) eth4.151 Traffic	Up	
78 kbit/s

02/09/2015 09:44:00	(013) eth4.151 Traffic	Notification Info	
State Trigger activated (Sensor/Source/ID: 17952/4664/1)

02/09/2015 09:43:36	Memory: Swap space	Up	
100 %

02/09/2015 09:43:08	(012) Mgmt.150 Traffic	Warning	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 09:43:08	Memory: Swap space	Down	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 09:43:08	(013) eth4.151 Traffic	Down	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 09:43:08	Memory: Swap space	Warning	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 09:43:08	(013) eth4.151 Traffic	Warning	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 09:13:23	(028) Mgmt.27 Traffic	Up	
0,01 kbit/s

02/09/2015 09:12:54	(027) eth4.251 Traffic	Up	
2 kbit/s

02/09/2015 09:12:27	(027) eth4.251 Traffic	Warning	
OK

02/09/2015 09:12:27	(028) Mgmt.27 Traffic	Down	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 09:12:27	(028) Mgmt.27 Traffic	Warning	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 08:43:08	(014) eth3.162 Traffic	Up	
217 kbit/s

02/09/2015 08:42:38	Memory: Virtual memory	Up	
79 %

02/09/2015 08:42:09	(014) eth3.162 Traffic	Down	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 08:42:09	Memory: Virtual memory	Down	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 08:42:09	Memory: Virtual memory	Warning	
SNMPERR_BAD_SENDTO (SNMP error # -12)

02/09/2015 08:42:09	(014) eth3.162 Traffic	Warning	
SNMPERR_BAD_SENDTO (SNMP error # -12)

.......

Thank you.