I have also been seeing 422 errors with no clear error message, just a generic Cart Error message.
This is an example request id: d303f122-3d13-49a2-8a56-f93bf2ca19cf-1769602454
The Cart API has been quite unstable in the past couple weeks, with also an incredibly annoying increase in 429 errors due to CloudFlare’s bot protection, which is particularly easily triggered when a customer is using a VPN.
EDIT:
The 422 error for that request id is due to the fact that the variant we were trying to add does not exist anymore. The error message could still be clearer on why the request failed.
The 429 errors are still an issue though 
(I have created a separate thread for it Aggressive bot detection with 429 errors AJAX Cart API)