UPDATE: This has been solved.
The issue was on our end and due to new topics.
Last Friday after publishing a new blog article, our blog landing page started logging 500 Internal Server Errors and wouldn't retrieve any of the blogs or topics. Saturday and Sunday, the Hubspot API Monitoring records showed all HTTP 200, then by Monday it started alternating between 200 and 401. Today while doing more testing it was showing all 200s.
After realizing that it was the last entry that was causing the issue, we pulled it and started experimenting. What we found is the following:
Starting a new blog entry from scratch always causes the errors.
Cloning a working entry and editing content and the URL also causes it to break.
Cloning a working entry but without editing the URL does work.
Right now I can't tell if this is a Hubspot issue or something with our site. None of the code that touches the API has changed, and the blog page works if looking up posts by topic or the search bar.
FYI, I'm not that familiar with HubSpot, so I'm sure I'm missing some details.