ffmpeg rtsp with port forwarding fail when specifying target port

[Gustavo Hylander]

Summary of the bug:
When using a pc as a bridge between the target source of rtsp stream, and the device where ffmpeg is executed, when specifying a port the execution will fail with error "Invalid data found when processing input". This does not happen if the port is removed (which leads to unauthorised).

How to reproduce:
The setup i have is a network camera. I have a pc that can, through a vpn, access said camera. I have a 3rd device where i execute ffmpeg. On the pc, i forward all traffic inconming to port 9000 to the camera's port 554. Ffmpeg will fail to connect to the camera this way.

ffmpeg -y -i rtsp://user:pass@IP:port -filter fps=fps=1 -updatefirst 1 /var/www/html/cache/raw.jpg   
ffmpeg version: 3.4.8
built on ubuntu

[Gustavo Hylander]

Updated ffmpeg to version 4.3.1 but the problem persists