If your environment is up and running, CI builds are running as expected, but you suddenly start getting 403 error responses with your builds, it may be due to the Temporary Login Suspension functionality.
Once a user login attempt fails to authenticate with 401 error, and these failures are consecutive within a short timeframe, then, due to the "Temporary Login Suspension" functionality, that user gets temporarily blocked for a short period (depending on the number of failures, up to 60 seconds).
During this timeframe, every request of this user, even with the correct password, will be blocked by Artifactory, with a 403 error.
If you have rotated your API Keys or Access Tokens lately, it might be the explanation for the 403 issue. Most likely there’s some agent or client that still uses the old API-key/token/password, which 'locks' the user temporarily, and during this timeframe, requests are failing with 403 errors.