Aha finally I can see a map!
The first error about gregorian calendar disappeared when I added a config option to specify localization.
<script src="./ags3-13-standard/dojo/dojo.js" data-dojo-config="async: true, locale: 'en-us'"></script>
Note however that the app should really be in en-gb, which did not work.
It still cannot found /dojo/dojo.js.map but so far that has not been a problem (with the vanilla app from developer sandbox).
Still any tips about this topic welcome.
Filip.