Server can't connect to Agent even though port 10050 is open

[Jamo]

Hey there.

I know that this question might sound a bit stupid because the whole "I can't get data from the Agent" thing has been asked at least 1000 times.
But, at least as far as I see, the problem always was that the client didn't allow connections on port 10050.

Well, in my case the client allows connections on port 10050. I've verified that with the help of a quick nmap scan.
Here you can see the responsible part in my iptables:

Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:zabbix-agent
ACCEPT udp -- anywhere anywhere udp dpt:10050

But even though the port is definetly open, my Server gives me the error message

Get value from agent failed: cannot connect to [[x.x.x.x]:10050]: [4] Interrupted system call

The agent is configured to work passively (default config) because I can't open ports in the firewall that's behind the Server (because my ISP is trash and doesn't allow me to do so ...)

Does anyone have an idea what else could cause the Server not being abled to get data from the agent?

Thanks in advance everyone.

[cyber]

Anything in Agent log.
You could run tcpdump and try to understand, what goes on...