I'll check on logs.

I found several open and closed upstream issues with Reeder and / or the GReader API:

https://github.com/FreshRSS/FreshRSS/issues/2759
https://github.com/FreshRSS/FreshRSS/issues/3052
https://github.com/FreshRSS/FreshRSS/issues/2684

So I switched to the Fever API that FreshRSS provides and it works now... (freshrss.mydomain.tld/api/fever.php)