UNKNOWN: session get request failed

Hello

I have 3 different fortigate

In two of them models 200B and 600C the plugins work perfectly for me

But on model 501E I get the following error UNKNOWN: session get request failed

The SNMP configuration is activated in all 3 fortigate the same, with the IP of my nagios server, with the SNMP events cpu-high mem-low fm-if-change activated, I also have the protocol v1 and v2c port 161 enable activated

Could someone help me, this is driving me crazy

Thank you