Hey,
I just noticed, that the url Tag fails at some special characters like Umlauts. If I want to link to this page e.g.:

https://de.wikibooks.org/wiki/LaTeX-Wörterbuch:_Anführungszeichen

and place it inside of the url tags it appears as follows:

[url=https://de.wikibooks.org/wiki/LaTeX-Wörterbuch:_Anführungszeichen]link[/url]
[url="https://de.wikibooks.org/wiki/LaTeX-Wörterbuch:_Anführungszeichen"]link[/url]
[url]https://de.wikibooks.org/wiki/LaTeX-Wörterbuch:_Anführungszeichen[/url]

It simply doesn't display the Links due to the Umlauts. If I escape the symbols though (replacing ü with %FC and ö with %F6) it works. I don't know how flexible phpBB is, but if you can find a way to escape special characters in URLs it should work.

RC-1140

Seems to be a phpbb issue:
http://tracker.phpbb.com/browse/PHPBB3-3981
I'll look into it if there's an easy fix.

Should be fixed, thanks for reporting.

That's an old one:
deskthority-related-f17/umlautz-in-links-are-aint-no-good-t546.html