Hi Jeff,
My first suspcion is that your C# application is using iFrame from IE -- and the default version Windows defines is old, and does not support the newer versions of High Charts. You can prove this by running IE 8 on your new site to see if the charts fail to display. You can force Windows to use a newer version of IE or Edge by changing some registry setting(s). Have a look at: https://social.msdn.microsoft.com/Fo...lating-msie-70 and https://docs.microsoft.com/en-us/pre...wser_emulation.

Basically, you have to add your application [name].exe and specify which browser version you want invoked went instantiating an iFrame: 11001 = IE version 11
Registry key to edit:
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Int ernet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULA TION
OR
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Int ernet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULA TION
OR
this is the one that worked for me.
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\M icrosoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULA TION

HTH,
Bob