I was having this same problem -- kept getting "this page is not available" on a 500 error URL (https://www.bing.com/webmaster/Error/Error500?aspxerrorpath=/webmasters).
Finally, I took drastic measures and deleted my cookies and site data for the site. MAGIC! Everything works again. I have no idea what totally hosed it, but clearing that out restored access. Hope this can help someone out.