Steps to reproduce
proc.num seems to have an issue with long process IDs on the latest Ubuntu LTS Release.
On Ubuntu 18.04 the check gives the correct result:
Same check on Ubuntu 20.04 gives:
Root cause seems to be a change in /proc/sys/kernel/pid_max:
On Ubuntu 18.04 it's 32768, in Ubuntu 20.04 it's 4194304. Looks like Zabbix Agent has an issue with bigger PIDs.