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:

# zabbix_agentd -t proc.num[fail2ban-server]
proc.num[fail2ban-server] [u|1]
# ps wax |grep fail2ban
590 ? Ssl 207:44 /usr/bin/python3 /usr/bin/fail2ban-server -xf start

Same check on Ubuntu 20.04 gives:

zabbix_agentd -t proc.num[fail2ban-server]
proc.num[fail2ban-server] [u|0]
# ps wax |grep fail2ban
3446386 ? Ssl 93:28 /usr/bin/python3 /usr/bin/fail2ban-server -xf start

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.