Yesterday I was runnig Tails and obviously using tor.
The point is that some subreddits doesn't allowed me to access, showing a 503 message. After a moment, the message appeared in general. Right now using tor on Debian, I'd tried to open the same posts and everything just worked fine.
I found this guy experiencing something similar
This also happens to me running Tor Browser 10.0.8 for Linux. I don't know why this is happening, but I have figured out a workaround: change the URL from https://www.reddit.com/r/... to https://old.reddit.com/r/...
Doing this allows me to access any of the pages on which I previously received a 503 error. Hopefully it works for you