Do you use a bookmark or are you starting with the main (home) page of this website?

If you use a bookmark to access a specific page, instead navigate to this page starting with the main page or with the sign in page in case there is a problem with this bookmark.

You can try these steps in case of issues with webpages:

You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings).

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

Thank you for your reply, cor-el.

I usully clear my history after I've finished work, but maybe it's not as thourough as the steps you mention but I clear everything with the option "from to start of the account" which means that I don't clear everything for only the last week, month or any other specific period. I have the brower in Swedish so I'll have to look try and translate your buttons to mine. But I think I can manage the "press" Shift, Ctrl and the other steps associated with those keys, as the top bulleted list (with three lines).

If I don't manage to sort this out I suppose I have to live with the step of pressing Backspace to get past the error msg. I wish I was better at computers, but there are other things I'm better at (not related to web browsers) :)