Saxman Said:
"Emails opening blank: [...]So yes, they are in my eMail account on the website, it's only on my Mac they disappear, sometimes as I'm in the middle of reading or replying to one!!"
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What to Do:
Being a continuum of errors, then here is what to consider....
A. Boot Into Safe Mode:
[Hold Down: the shift key upon boot]. Keep holding it down until you reach the log in screen. Then, log in, wait 30 seconds, and then boot up normally. In Safe Mode, damaged system files (i.e. caches) are fixed, and replaced.
B, Clearing your Temporary Internet Files (Cookies & Cache) of Safari:
Cookies perform remembrance of certain site-entered data. Whereas Cache keeps a snapshot of the last time you visited a site. Use this Link: Clearing History and Cache - Apple Communities
C. Try New Admin User:
On your Mac. try creating a new Administrator User, and see if you can configure it with the new user. If it then works, then it is likely something configured in your current user. Delete the user when all is fixed.
D. Reinstall the MacOS:
So, as a last resort, reinstall the macOS. You would backup your Mac, and then reinstall it. Nothing would be removed. It's just that items that have been moved or corrupted, would be placed back to where they should be or replaced, respectively. But, more on that later, if it comes to that. I look forward to hearing back from you. Keep us up to date. Important: Create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the reinstall. Use an external drive at least 1.5 times the size in storage capacity.
E. Time Machine Backup Restoration:
Have a backup that you created prior to all of this happening? If so, restore your Mac from it.