Sending data from Teltonika FMB003 to Traccar behind Nginx

Then I don't understand, why it's only listening on IPv6 ports on my machine. I just reinstalled it and it didn't change anything. Perhaps something in the Java config (which is untouched by me)?

Probably you misunderstand the stack. Modern systems have universal dual stack. For example I see something like this on our servers:

tcp6       0      0 :::5074                 :::*                    LISTEN     
tcp6       0      0 :::5042                 :::*                    LISTEN     
tcp6       0      0 :::5106                 :::*                    LISTEN     
...

It does not mean that it only listens on IPv6. It handles both and we have mostly IPv4 data coming in just fine.

I found the problem. My hosting provider blocked the port via IPv4 but not via IPv6...