The same issue as the .js file. Not related to Mx5.15 itself. I have it with Mx5.14.1 and Mx5.15
The .css is named Calendar.css, should be calendar.css. Because of that, it is not included in the widgets css.
If you are in a hurry: Until a new distribution is made, open the Calendar.mpk file using WinRar or similar and rename the Calendar.css to calendar.css in the archive. Deploy again and it is OK.
Thnx Marcel! I have changed the name of the css and it works. Calender is rendered as usual...