The toolbars in the Webviewer application use fonts to paint the labels on the buttons and, in IE settings, we found two (2) settings that would break the Webviewer presentation and cause the buttons to appear to be blank.
It seems that some corporate users may have their default settings changed by their IT department. The 2 settings in Internet Explorer are:
General > Accessibility > Ignore font styles specified on web pages. When enabled will block the Webviewer fonts from being loaded. The setting needs to be disabled.
2. Security > Customer Level > Downloads > Font Download. When disabled also prevents the Webviewer fonts from loading. This setting needs to be enabled to allow the fonts to be downloaded and the text to then appear.