Steps to replicate the issue (include links if applicable):
Enter {{fullurl:{{FULLPAGENAME}}|useskin=monobook}} onto an wikipage
Same with {{SERVER}} and {{filepath:}}
What happens?:
An proto-relative URL is generated.
What should have happened instead?:
An URL with the https or http protocol should have been created according to
https://lists.wikimedia.org/hyperkitty/list/wikitech-l@lists.wikimedia.org/thread/USZRHK5NJRHSFBWD6Q6RZQGS2R4E4BZ7/
Software version (skip for WMF-hosted wikis like Wikipedia): WMF
Other information (browser name/version, screenshots, etc.): Irrelevant, server side error.
Related: T312666