@reachstars
It sounds like there is some fundamental underlying issue, either with your network or your account, that has been adversely affected by recent eBay changes starting around June, and further complicated by the new sign-in page.
Unless you can point to some aspect of your account or network that makes it different (using a VPN? Unique firewall or antivirus settings?) or you can find someone else having the same ongoing issue, I don't think there's much a random user on a forum can do other than make suggestions of different things to try and rule out.
To rule out network issues: Start with a browser you don't normally use (so you can play with settings and not screw up your current account), go to ebay.com and create a new account. If you can successfully log in and navigate without problems, I think that would indicate your network is fine.
Alternatively, if you have access to a different network, try signing on to your current account and see if it works. Start at ebay.com rather than relying on a bookmark.
Consider changing your my ebay landing page -- in theory, if your landing page were screwed up enough, it could be kicking you back into a sign-on page and creating a loop. Try something generic like your watch list or summary rather than seller hub.
Normally I would say to try changing your password -- but in your case I would be worried that you would never get back into your account. Might as well save that for when you can no longer get in at all.
Anybody else have any ideas?
