I’ve noticed that when I import and convert old files from Ardour 4, some tracks will only play on the left channel and the Left-Right slider of the mixer is stuck in the middle. All tracks were recorded in mono in Ardour 4. When I right click on the tracks and select “Outputs”, I can see “L” (left) “R” (right) and “1”, which seems to be a “port”.
Yes, just the one file should be sufficient to investigate.
You could try if it fits on https://pastebin.com/ otherwise perhaps google-drive, drop-box or similar service… If you do not have access to any of those services, you could attach it to a bug report at https://tracker.ardour.org/ (but that requires a dedicated account)
This indicates a stereo track, you first have to reduce the width, before you change the center – The Ardour Manual
Alternatively right-click and change it to a stereo-balance control.
I see, you have loaded a stereo plugin onto the mono track. Specifically Calf Reverb. I expect you have not only updated Ardour, but also Calf plugins. Recent Calf adds a MIDI output, which explains why you’re seeing the “1” in addition to L+R.
The easy solution remove the plugins from each track.
Then add a stereo bus, load a revert plugin there, and use sends to the reverb-bus.
It is usually preferable to use a single Reverb, that ties the mix together like it was recorded in a single room. Individual reverb effects on tracks do not necessarily achieve that.
Another solution is to enable “strict i/O” (context menu of the mixer-strip header)
This prevents plugins from adding ports and the mono track will remain mono. However I do not know if that works well with Calf Stereo Reverb.