With Camponez's help, I made an account on his wiki and verified that everything appears to be set up correctly. I found that the error will go away if I remove the validate:int command, replacing "<!--{$id|escape:'html'|validate:int}-->" with "<!--{$id|escape:'html'}-->."
For some reason, despite the fact that we have exactly the same text in Widget:SoundCloud, the exact same widget call works on my site but fails on his. However, other widgets work just fine for him. This is illustrated on the following page:
http://www.cruzeiropedia.org/Usu%C3%A1rio:ForestNote that the Twitter widget works but the SoundCloud widget doesn't.
Our stacks our quite similar:
Smarty 3.1.21
Thoughts from developers? Now we have three people who have separately identified a problem.
For the record, the widget is usable if you remove the validate:int command from the id variable. However, the id variable is indeed an integer, so it should pass validation:
{{#widget:SoundCloud
|id=16040214
|width=100%
}}
Forest