If viewing a specific calendar (e.g. the preinstalled "Other Events" calendar, then below the calendar widget some XWiki markup is displayed:

{{/html}}
{{/html}} ))) 

 
Seemingly some html macros have not been closed properly.

This problem must have been "swallowed" in earlier XWiki versions; so far it has been observed in XWiki 14.10.19, 14.10.20 and 16.0.0.