ffmpeg: 'Protocol not found' for normal file name

I came to this topic for the message, just to add one tip for users, I found out that if the filename itself has ':' in it would cause the problem, as some programs generate files with the time stamp like "audio 12:34:14.ogg"

The comment of @andcoz lead me onto the right way: the problem exists as long as /usr/lib/chromium-browser/libffmpeg.so is loaded.

In my case this happens in some special use case:

The libffmpeg.so is used by the Vivaldi browser for showing proprietary video formats (H.264).
If I download a file via Vivaldi, and then go to its download panel, right click on the downloaded file and select 'Show in file manager', my file manager opens. It runs in the 'context' of the browser, so when I go a step further, and open a terminal out of that file manager session, this also runs in the context of Vivaldi.
For me this means, that there are a few environment variables set, which are not there, if I start a normal terminal session (not out of Vivaldi context).
These environment variables contain (among others) a LD_PRELOAD=/usr/lib/chromium-browser/libffmpeg.so. As that one causes libffmpeg.so to be loaded when running my conversion with ffmpeg, I tried an unset LD_PRELOAD in the terminal session and after that my conversion ran again fully normal.

So this is where the 'then and now' problem came from, I just was not aware of the case I am in a different environment variable context when starting my program chain out of Vivaldi.

Thx everyone for helping out.