Instead of clearing cache, there's another way you can test to see if the problem is on the server end. Open a private/incognito window in your browser - this doesn't use the same caching data, so it's "refreshed" in a sense. If you can log into QBO without any problems, it's not your cache. It may not necessarily be a server problem on our end, but you (probably) don't need to clear anything in your regular browser window.
A simple fix that goes a long way in fixing lots of web-based programs (QBO included).