See if this link helps; https://www.bing.com/search?q=Internal%20Server%20Error&pc=MOZI&form=MOZSBR

This issue can be caused by corrupted cookies or cookies that are blocked. *check the permissions for the domain in the currently selected tab in "Tools -> Page Info -> Permissions" *https://support.mozilla.org/en-US/kb/fix-login-issues-on-websites-require-passwords Clear the Cache and remove the Cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences). "Remove the Cookies" from websites that cause problems: *Options/Preferences -> Privacy & Security<br>Cookies and Site Data -> Manage Data "Clear the Cache": *Options/Preferences -> Privacy & Security<br>Cookies and Site Data -> Clear Data -> Cached Web Content: Clear ---- You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History"). Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note. You can't recover from this 'forget' unless you have a backup of involved files. If you revisit a 'forgotten' website then data from that website will be saved once again. *https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

Many Thanks

Many Thanks