Thanks for getting back to me, Nikko.

In the meantime, we managed to trace the culprit down to code in .htaccess which was causing the conflict.

We removed the following:

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<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
#RewriteCond %{HTTP_HOST} ^www.advsolned.com [NC]
#RewriteRule ^(.*)$ http\:\/\/advsolned\.com\/$1 [R=301,L]
# Start WPFC Exclude
# End WPFC Exclude
# Start_WPFC_Exclude_Admin_Cookie
RewriteCond %{HTTP:Cookie} !wordpress_logged_in_[^\=]+\=admin
# End_WPFC_Exclude_Admin_Cookie
RewriteCond %{HTTP_HOST} ^advsolned.com
RewriteCond %{HTTP_USER_AGENT} !(facebookexternalhit|WhatsApp|Mediatoolkitbot)
RewriteCond %{HTTP_USER_AGENT} !(WP\sFastest\sCache\sPreload(\siPhone\sMobile)?\s*Bot)
RewriteCond %{REQUEST_METHOD} !POST
RewriteCond %{HTTPS} !=on
RewriteCond %{REQUEST_URI} !(\/){2}$
RewriteCond %{REQUEST_URI} \/$
RewriteCond %{QUERY_STRING} !.+
RewriteCond %{HTTP:Cookie} !wordpress_logged_in
RewriteCond %{HTTP:Cookie} !comment_author_
RewriteCond %{HTTP:Cookie} !wp_woocommerce_session
RewriteCond %{HTTP:Cookie} !safirmobilswitcher=mobil
RewriteCond %{HTTP:Profile} !^[a-z0-9\”]+ [NC]
RewriteCond %{HTTP_USER_AGENT} !^.*(\bCrMo\b|CriOS|Android.*Chrome\/[.0-9]*\s(Mobile)?|\bDolfin\b|Opera.*Mini|Opera.*Mobi|Android.*Opera|Mobile.*OPR\/[0-9.]+|Coast\/[0-9.]+|Skyfire|Mobile\sSafari\/[.0-9]*\sEdge|IEMobile|MSIEMobile|fennec|firefox.*maemo|(Mobile|Tablet).*Firefox|Firefox.*Mobile|FxiOS|bolt|teashark|Blazer|Version.*Mobile.*Safari|Safari.*Mobile|MobileSafari|Tizen|UC.*Browser|UCWEB|baiduboxapp|baidubrowser|DiigoBrowser|Puffin|\bMercury\b|Obigo|NF-Browser|NokiaBrowser|OviBrowser|OneBrowser|TwonkyBeamBrowser|SEMC.*Browser|FlyFlow|Minimo|NetFront|Novarra-Vision|MQQBrowser|MicroMessenger|Android.*PaleMoon|Mobile.*PaleMoon|Android|blackberry|\bBB10\b|rim\stablet\sos|PalmOS|avantgo|blazer|elaine|hiptop|palm|plucker|xiino|Symbian|SymbOS|Series60|Series40|SYB-[0-9]+|\bS60\b|Windows\sCE.*(PPC|Smartphone|Mobile|[0-9]{3}x[0-9]{3})|Window\sMobile|Windows\sPhone\s[0-9.]+|WCE;|Windows\sPhone\s10.0|Windows\sPhone\s8.1|Windows\sPhone\s8.0|Windows\sPhone\sOS|XBLWP7|ZuneWP7|Windows\sNT\s6\.[23]\;\sARM\;|\biPhone.*Mobile|\biPod|\biPad|Apple-iPhone7C2|MeeGo|Maemo|J2ME\/|\bMIDP\b|\bCLDC\b|webOS|hpwOS|\bBada\b|BREW).*$ [NC]
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/all/$1/index.html -f [or]
RewriteCond /home/advanced/public_html/wp-content/cache/all/$1/index.html -f
RewriteRule ^(.*) “/wp-content/cache/all/$1/index.html” [L]
</IfModule>

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Not sure what this code is meant to do, it all seems to work now.
Nikko, perhaps you can provide some input here?

Regards,
Sanjeev.