I suspect that it has happened after uprgades to Firefox etc as I can still make it work on Safari under windows and partially on a very old copy of Opera.
The print page is opened (on separate tab) and then just displays the GIF image.
Firefox 50.1.0 running on Windows 7 not working.
Chrome (and Derivatives) not working
Safari old PC version (latest works fine
The fix
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After opening the Developer Console it looks like the site is delivering mixed content (http / https ) and I suspect that it is because the latest browsers are defaulting to HTTPS for all sites. Even if the main site is opened with HTTP: the print ends up as HTTPS: and you cannot just change the address to HTTP: and refresh the page.
In Firefox you can get the page working by clicking the i button to the left of the address and the arrow to the right and then Disable Protection for Now.
In Chrome you can get the page working by clicking the shield to the right of the address
I cannot work out how to get it working in Safari on the IPAD (with IOS 9) and some other browsers.
Developer Console (Chrome) shows these errors
Mixed Content: The page at 'https://wtp2.appspot.com/print2.htm' was loaded over HTTPS, but requested an insecure script 'http://maps.google.com/maps/api/js?sensor=false&libraries=panoramio,geometry,weather'. This request has been blocked; the content must be served over HTTPS.
Mixed Content: The page at 'https://wtp2.appspot.com/print2.htm' was loaded over HTTPS, but requested an insecure script 'http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js'. This request has been blocked; the content must be served over HTTPS.
followed by a series of Uncaught ReferenceErrors e.g.
jquery_cookie.js:10 Uncaught ReferenceError: jQuery is not defined
at jquery_cookie.js:10
(anonymous) @ jquery_cookie.js:10
warped_os.js:101 Uncaught ReferenceError: google is not defined
at warped_os.js:101