Make sure your browser is up-to-date and has JavaScript enabled (and not blocked):
https://www.whatismybrowser.com/
In some cases, if you are using a bad link and getting stuck in a redirection loop, you can get that error message.
If the problem happens when trying to sign in, see if you can use a different link to sign in.
https://signin.ebay.com/ws/eBayISAPI.dll
If you are using a mobile browser, try changing to a different browser mode: mobile, classic or desktop. Sometimes one page may not render properly in mobile or classic mode, and that may cause a redirection to a different mode, but the page may redirect back to the first page in a loop. Try a desktop browser or a mobile browser in desktop mode.
Or try signing in to your usual account using the eBay app. If you see that message while using the eBay app, it means eBay does not recognize your device's default browser; try changing it to Firefox.
