Thanks. The Firewall rules are permitting SNMP v1 and v2 from what I see. So you will want to ensure that the Fortigate is permitting SNMPv2.c with the correct community string. I'd suggest testing with the Lansweeper Device Tester app (C:\Program Files (x86)\Lansweeper\Actions\Devicetester.exe). If that passes, then create a new set of credentials with the successful username and password and assign it to the new scan server. I'm not too familiar with Fortigate but I'm assuming you should be able to console into the unit and run a debug for SNMP to see if the unit is rejecting access from the New Scan Server while you are testing. Would it be possible that Fortigate has a limit to the number of devices that can communicate with a specific SNMP string? May be worth considering creating new credentials for testing/implementation.
Alternatively, you may also use "Paessler SNMP Tester" for testing. I will not post the link here.
I've located the below KB that may be of help in testing SNMP.
Fortigate KB 195334
If the above information does not help, then I would suggest reaching out to Lansweeper support directly by emailing them at support@lansweeper.com. Here is their KB containing what information they'll need from you. It would probably be helpful for you to apply the below settings to your environment via the Lansweeper Config Editor ("C:\Program Files (x86)\Lansweeper\Tools\ConfigEditor.exe")(Within the app, click Advanced at the top and then import app settings) and then test before you open the case so that they have all their desired information, thus minimizing your wait time. Though, you may want to wait on their instructions as they are the professionals and I'm just an end user.
<?xml version="1.0" encoding="utf-8"?>
<exportDebug>
<appSetting>
<key>Debug</key>
<value>1</value>
</appSetting>
<appSetting>
<key>LogSnmp</key>
<value>7</value>
</appSetting>
<appSetting>
<key>LogSnmpPass</key>
<value>7</value>
</appSetting>
<appSetting>
<key>LogDeviceDuplicates</key>
<value>7</value>
</appSetting>
<appSetting>
<key>LogSsh</key>
<value>7</value>
</appSetting>
<appSetting>
<key>LogIpMain</key>
<value>7</value>
</appSetting>
<appSetting>
<key>LogDeviceUpdate</key>
<value>7</value>
</appSetting>
</exportDebug>
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LS Tech Support Email: Support@lansweeper.com
LS Tech Support KB: https://www.lansweeper.com/contact-support/