Steps to reproduce:

1. Installed agent2 behind zabbix proxy on a host on internet. Proxy is ACTIVE.
2. Create a template with a key, "exim.paniclog" here
3. Create a file /etc/zabbix/zabbix_agent2.d/exim-paniclog.conf with UserParameter, contents:

UserParameter=exim.paniclog,[ -f /tmp/paniclog ] && cat /tmp/paniclog

4. Add template to regular agent on a VM available within LAN. It works correctly.

5. Create a remote VM behind a proxy. It works correctly.

6. Add a template and a UserParameter file to remote VM.

Result:

Agent on remote VM stops working. Log messages:

Proxy:

4092:20201202:171737.447 received configuration data from server at "example.com", datalen 11556
4111:20201202:171748.429 Zabbix agent item "exim.paniclog" on host "emx2" failed: first network error, wait for 15 seconds
4114:20201202:171803.443 Zabbix agent item "proc.num[]" on host "emx2" failed: another network error, wait for 15 seconds
4114:20201202:171818.448 Zabbix agent item "system.cpu.load[percpu,avg1]" on host "emx2" failed: another network error, wait for 15 seconds
4114:20201202:171833.452 temporarily disabling Zabbix agent checks on host "emx2": host unavailable

Agent

2020/12/02 17:19:48.171532 Starting Zabbix Agent 2 (5.2.1)
2020/12/02 17:19:48.172560 OpenSSL library (OpenSSL 1.1.1d 10 Sep 2019) initialized
2020/12/02 17:19:48.172864 using configuration file: /etc/zabbix/zabbix_agent2.conf
2020/12/02 17:19:48.173112 cannot initialize user parameters: cannot register user parameter "exim.paniclog,[ -f /tmp/paniclog ] && cat /tmp/paniclog": key already used

I've read Release Notes for Zabbix 5.2.2, did not find anything about some proxy related fixes.

Expected:

Agent on remote VM can execute UserParameter checks.