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This issue might be caused by header_sent php error, have you made modifications to your functions.php file? or other php files? any new plugin?

I just updated firefox and adobe flash player… could that have something to do with it?
It did say the same thing when i opened it in explorer…

I don’t think this is a browser issue, more a website issue! please check my above reply.

I have worked on the website the last few days and installed a few new plugins. I dont’t think that I have made any modifications to the funtions.php file (I barely know what that is…)

Plugins can cause this, you’ll need to disable them to make sure they are not the ones to blame for this. Here is how you do it without admin access:
https://www.wpbeginner.com/plugins/how-to-deactivate-all-plugins-when-not-able-to-access-wp-admin/

I recently moved my website to a new service and ran into the same issue. I followed the advice above and disabled one plugin at a time. After disabling ‘Simple Backup’ everything started to work again.