Steps to reproduce:
Currently not possible to reproduce it specifically. It occours randomly on 1% of 7000 hosts per week. Doesn't occour again on the same host after debug log was enabled and I would prevent to enable debug logging in all 7000 hosts.
So lets describe the environment:

• all hosts are Debian-based Proxmox hosts of different versions
• all have Zabbix Agent 2 Version 6.4.11, Server and Proxys are 6.4.11 also
• only 2 Agent 2 Plugins are affected: Cpu and VFSDev
• round about 120 agent items per host, mostly built-in item keys with intervals between 5 and 15 minutes

• • for Cpu plugin following item keys are used:
    • system.cpu.util
    • system.cpu.util[,iowait,avg1]
    • system.cpu.util[,idle,avg1]
    • system.cpu.num
  • ZabbixAsync Plugin with Item key system.cpu.load is still working, like all other item keys
• normally all capacity counters are 0
  • I tried it also with a stress test and call the affected item keys at working time with multiple threads thousands times but it worked well.
• seems that there is a special scenario why the agent doesn't free up the threads

Checked also:

• no memory leak, working and broken agents needs round about 30MB RSS memory
• threads don't clean up after time, if there are no new requests to the agent (blocked via iptables for debugging)
• open files count far away from any limit

Result:
capacity and tasks of affected plugins fills up. Metrics output:

[Cpu]
active: true
capacity: 100/100
check on start: 0
tasks: 6484
system.cpu.discovery: List of detected CPUs/CPU cores, used for low-level discovery.
system.cpu.num: Number of CPUs.
system.cpu.util: CPU utilization percentage.

[VFSDev]
active: true
capacity: 100/100
check on start: 0
tasks: 220
vfs.dev.discovery: List of block devices and their type. Used for low-level discovery.
vfs.dev.read: Disk read statistics.
vfs.dev.write: Disk write statistics.

proxy1 # zabbix_get -s <ip> --tls-connect psk --tls-psk-identity "Agent_m16346" --tls-psk-file /etc/zabbix/zabbix_agent.psk -k system.cpu.num
zabbix_get [2553466]: Timeout while executing operation
proxy1 # zabbix_get -s <ip> --tls-connect psk --tls-psk-identity "Agent_m16346" --tls-psk-file /etc/zabbix/zabbix_agent.psk -k system.cpu.load
57.280000

Debug logs show no helpful information after capacity limit is reached already.

Expected:
Empty cacpacity and working items

Additional:
Any hints what I could do, to give you more details?