There are several current threads on the Community Platform Feedback board and elsewhere about problems signing in to the community pages or staying signed in. Sometimes using the sign-in link seems to just reload the main Community page without actually signing in or even asking for credentials.
There are many proposed workarounds such as using different community links, signing out of eBay altogether or clearing eBay cookies, making sure JavaScript is enabled in your browser, opening the "sign-in" or "join" link in another browser tab, signing in using a private or incognito browser window, or using various account features such as loading a profile page, replying to a message or providing a helpful vote that seem to result in being signed back in, but what works once may not work again the next time it happens.
