Turning off the real-time scanner would be a very bad idea. Better create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

Also see https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

Creating an exception in Thunderbird (TB) (Tools/Options/Security uncheck allow antivirus clients to quarantine...) did not solve problem. Only unchecking Scan Inbound Mail in Avast/Mail Shield or turning off mail shield entirely allows mail to be received. Suggestions?

Attempted multiple reboots and cleaning of caches and registry with CCleaner. Nothing works.

Oddly, TB was working fine for years up until a few days ago. Either TB and/or Avast has an update that makes the Mail Shield for incoming POP3 mail not to work.

Creating an exception in Thunderbird (TB) (Tools/Options/Security uncheck allow antivirus clients to quarantine...) did not solve problem.

I was talking about creating an exception in Avast, not in Thunderbird.

Only unchecking Scan Inbound Mail in Avast/Mail Shield or turning off mail shield entirely allows mail to be received.

'Unchecking Scan Inbound Mail' sounds about right. I have no idea about Avast, and I don't know what 'turning off mail shield entirely' means. In general, Avast shouldn't mess with email at all.

Attempted multiple reboots and cleaning of caches and registry with CCleaner.

You're waisting your time. Wrt CCleaner, don't let CCleaner mess with Thunderbird stuff. It will only make things worse.