XnView Software

Thanks for your suggestion.

Problem solved by doing the following:
Uninstalled XnView
Restarted
Ran Regedit and removed all instances of XnView
Restarted
Installed XnView
Ran XnView as Admin
XnView Options/Windows File Associations
Set Xnview as default App.

Now works perfectly so as you say it ...

xnview wrote:if you put these 2 files in a folder, you can view them in XnView's browser?

Confirm can see both files in a browser folder.

It is only when I double click a tiff file that nothing shows

XnView 2.43 Classic set as default for all photos

Attached two files:
Tif File.tif – MD5: ce9bbc54c4700c649aa7a3957957ac56
Tiff File.tiff – MD5: ce9bbc54c4700c649aa7a3957957ac56
Both files are identical except for extension & name

When I double click either file in File Explorer:
The Tif File.tif ...