I do not have any information about eBay Partner Network, but if you are having problems signing in to eBay, try using a different browser -- preferably a recently updated one with no add-ons or extensions, using default settings aside from possibly disabling tracking protection.
Some sign-in issues can be caused by your browser cache or cookies, so clearing those might be helpful in some cases, particularly if you are able to sign in using a different browser.
Instead of using a bookmarked link, try going to ebay.com and signing in there directly.
