Possible Reasons:
MIME types not set
Missing project files
No startup screen set (Secure Viewer)
ISSymbol.ocx has been blocked from uploading or updating I.E. (Win 8 Security)
Solution:
Configure the MIME types in IIS
Republish the project
Run ThinClientSetup.exe on the target machine to manually load the ISSymbol.ocx, or start I.E. under Run As Administrator to update the plug-in, then restart normally.
MIME types not set
Missing project files
No startup screen set (Secure Viewer)
ISSymbol.ocx has been blocked from uploading or updating I.E. (Win 8 Security)
Solution:
Configure the MIME types in IIS
Republish the project
Run ThinClientSetup.exe on the target machine to manually load the ISSymbol.ocx, or start I.E. under Run As Administrator to update the plug-in, then restart normally.