@branquinho 

If rtsp streaming does not work, you could also try the low quality stream like stream 2.

If the rtsp has worked fine before, you could try to reboot or factory reset on the tapo camera. 

The error can have a specific meaning based on the app itself ,  it is suggested checking with some community of the RTSP or ONVIF apps and see if there are interpretations or suggestions.  

Good day.  @Antharian 

Thanks for sharing the insights. Ideally, it is recommended to keep 2 video streams on the camera , including both RTSP streaming and Tapo app live-streaming.

Here is related QA in part3:  

Q3.3 Can multiple accounts/devices view the Tapo camera at the same time?