Even if you have cleared your cache cookies in the past, if you are now having these issues with logging in, clearing them now may help with that.
Websites store cookies on your computer after you visit the sites to customize what you see on the page; if you have a cookie that has been corrupted, you may have trouble staying logged in. Also if the problem is related to a cookie on the website, it wouldn't affect the function of other things on the internet or on your computer-- it would only affect this website.
Clearing the cache at the same time will ensure that you aren't seeing old page information. Since you have had site issues, this will ensure that you aren't seeing old error messages when you shouldn't.