Found in GWT Releases 2.6.0 and 2.5.0: Encountered on Windows 7 / Google Chrome 43.0: When Google Chrome Debugger is open, no gwt project starts properly because file <PROJECT_NAME>.nocache.js tries to start undefined.cache.js, which is not provided. When I deeply investigated the problem, I found as follows: navigator.userAgent in Google Chrome 43.0 returns Mozilla/5.0 (compatible; MSIE 10.0; Windows Phone 8.0; Trident/6.0; IEMobile/10.0; ARM; Touch; NOKIA; Lumia 520) which is not recognized by the gwt startup code (2.6.0) and improperly recognized by gwt (2.5.0) (it thinks it is ie6).

Work-arounds: 1) If possible, do not open the debugger when gwt is loaded 2) if it is still necessary to open the debugger from the very beginning (this is my case because I need to track initial errors or network activity), manually change <PROJECT_NAME>.nocache.js each time after the gwt compilation, which is not convenient. It would be good, if the gwt startup also recognizes this situation.

Additional info: this navigator.userAgent returns such info only when Google Chrome Debugger is open, in other cases navigator.userAgent returns a normal information, which is properly recognized by gwt. In case of GWT 2.5.0 because of improper recognition (gwt thinks the browser is IE6), the exception is thrown: cannot be created because of illegal characters. But it is only the side effect of the improper recognition described above.

The altered user agent string is probably due to some configuration of your DevTools: https://developer.chrome.com/devtools/docs/device-mode You're expected to know what you're doing when using the devtools; this is not something that can be "fixed" in GWT, you're explicitly (even if unknowingly) tricking GWT into thinking you're using another browser (namely IE10 on Windows Phone 8 on a Nokia Lumia).

BTW, the issue tracker has now moved to GitHub: http://www.gwtproject.org/lifeofanissue.html