If you click on the link in [[User:Mr. Stradivarius/Sandbox/Previewbug]] you can see the error message, but if you edit [[Module:User:Mr. Stradivarius/previewbug]] and then use "preview page with this template" with "User:Mr. Stradivarius/Sandbox/Previewbug", the error message does not appear.
However, this bug does not occur on the test2 wiki. I've made equivalent sandbox and module pages at:
The error message displays fine with these pages. Because of this I'm not sure where the bug is. But according to , en.wiki and test2 are running the same version of Scribunto, so it's not a delay between patching one and patching the other. Therefore I'm submitting a bug here in case it is a software-side problem.