It seems something has changed in 3.21 regarding rate limiting. Acquisition used to load 60 tabs before throttling. Now it loads 31 and the next few times the number of loaded tabs is constantly reduced until it doesn't load anything any more.
It seems retrying too often causes this.
I contacted support but (as expected) didn't get any help regarding any changes to their APIs, just a URL that basically says their policies can change at any time without notice.
https://www.pathofexile.com/developer/docs#ratelimits