The solution is simple.
In the \bin directory on your DNN website is a file called: App_SubCode_HTMLService.dll
After backing up this file (just to be safe), then delete the file App_SubCode_HTMLService.dll from the website. This
should fix the issue.
Because the DNN upgrade to more recent DNN 5 versions also changed a compiled
test/html editor from an un-compiled version. The file we are removing is a
conflicting file that is left over from earlier versions.