Things to check: - Open the same page in another browser. - Open the same page in the private mode of the same browser (similar to clearing cache). - Open the Developer Console (I use Firefox) and watch if the page makes requests to those tiles or whether those requests are fulfilled. - If it's possible, comment out everything after loading the basemap then test again. - (You probably already doing this but) Make sure that loading the basemap is done after the page is loaded and the DOM is ready. Use Dojo's dojo/domReady!
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