Connection is being blocked by a firewall app like Little Snitch or similar.
Connection is being blocked by a URL filter on your router.
Twitch ingest servers currently unavailable-- you tested at least four, presumably in your region. You might want to try some outside of your region, to see if Twitch itself is having a problem.
Your ISP is intentionally blocking you from streaming by blocking Twitch ingest servers.
Does your computer have more than one network card? If so, is OBS perhaps configured in the Advanced tab to use a card that does not have Internet access? That might cause this problem.
The specific error message in the log is triggered either by an incorrect stream key or poor network connectivity:
Thank you so much for the ideas!
I tested again this morning without having changed anything, and it connected fine. I feel silly for having made a big deal over it now.
I'll keep these ideas in mind in case I have similar problems again though. Thanks again!