For the input {{#interwikilink:rfc|1{{Zeichen*|#}}appendix-I|1}}. The template actually outputs something like this

{{#interwikilink:rfc|1
    1. appendix-I|1}}

There's a new line at the beginning of the template output because the first character is #, and subsequently that starts a list item. The combination makes an invalid title, although it shouldn't crash this way.

Change #1102328 had a related patch set uploaded (by Ammarpad; author: Ammarpad):

[mediawiki/core@master] parser: User TitleParser to safely create TitleValue

https://gerrit.wikimedia.org/r/1102328

In T381977#10397385, @Ammarpad wrote:

For the input {{#interwikilink:rfc|1{{Zeichen*|#}}appendix-I|1}}. The template actually outputs something like this

{{#interwikilink:rfc|1
    1. appendix-I|1}}

There's a new line at the beginning of the template output because the first character is #, and subsequently that starts a list item. The combination makes an invalid title, although it shouldn't crash this way.

Arrrgh, yeah, sorry, that is the bloody *#;: behaviour of template transclusion.

However, the resulting page should have been regular output and perhaps a mistake or error message rather than Wikimedia\Assert\ParameterAssertionException error page, if a newline happens to occur in parameter value. I guess this will be remedied by the recent patch.

Change #1102328 merged by jenkins-bot:

[mediawiki/core@master] parser: Handle invalid titles gracefully in parser functions

https://gerrit.wikimedia.org/r/1102328