Here is the problem: Because this works with wav files, looks like REAPER can't read .ogg files generated by Free audio converter 1.0.26, although these files can be read by foobar. My version of REAPER is 5.28 for Windows 64.

Reaper v5.29 here. It reads .ogg files without problems.

Could not import items 

I see it's an old topic. But until recently I had no problems with importing ogg files. Now I make imports using Elevenlabs API, and the script returns files in ogg which Reaper doesn't understand. I have to convert to Mp3 or other formats before importing to REAPER. I suspect that Elevenlabs returns some crooked ogg. Maybe there is some fresh codec for Reaper that can import these files?

.ogg reading seems fine in REAPER here. If you share one of the .ogg files you are having trouble with, we can see why it's not being imported.