See parent bug (T246879) for the original report. (Note that the parent bug has been fixed with a work-around.)
It seems that if a local Mediawiki message includes {{urlencode|something}}, it will break the client-side message parsing.
Here's the actual code that is being called on the client-side:
mw.message( 'file-deleted-duplicate', filename ).parse()
| Status | Subtype | Assigned | Task | ||
|---|---|---|---|---|---|
| Resolved | kaldari | T246879 Upload Wizard gives blank templates rather than deletion log details | |||
| Invalid | BUG REPORT | None | T260488 Use of {{urlencode}} parser function breaks client-side message parsing |
Assuming this task is about the MediaWiki-Parser project, hence adding that project tag so other people can also find this task when searching via projects.
This is already documented as not supported: https://www.mediawiki.org/wiki/Manual:Messages_API#Feature_support_in_JavaScript