A cookie is information stored on your computer by a website you visit. The databases accessed through the Library catalogue use cookies, so they must be enabled in your browser. Many sites report a 400 error when a cookie it's reading is corrupt or too old.
Sometimes, problems with any webpage can be solved by clearing your browser cache.
Caching errors are most simply diagnosed by copying the URL that gives you the error and pasting it into a New Private or New Incognito window. If it works, then the issue is caching, history and/or cookies.
Clearing your cookies/cache/web history resolves a number of different issues and errors, including: