This was originally posted on APC forums on 9/25/2020
Hi, found the answer myself after comparing the Config-Files from affected and unaffected NMC2s. The only difference (beside the MAC, IP, Names...) are the TrapReceiver5Community=public and TrapReceiver6Community=public entries.
The affected (generating automated pings to the gateway) NMC2 have the following conf lines in the [NetworkSNMP] block:
I copied the whole [NetworkSNMP] block from an unaffected NMC2 to a new config-file (ini) and imported this file (with only the [NetworkSNMP] block) in the affected NMC2. After this and rebooting the NMC2, no pings are generated anymore.