💡 Found that the .htaccess file in public_html folder on the server had been overwritten which caused the problem.
Added this code obtained from the IndieWeb -> IndieAuth plugin on WordPress backend into the .htaccess file to resolve the syndication problem I was experiencing. IndieAuth and micropub back up and running:
SetEnvIf Authorization "(.*)" HTTP_AUTHORIZATION=$1
I had probably added that code into an overwrittable part of the file previously.