I think it may be related to T36873

In T15387#173238, @Nemo_bis wrote:

Works now it seems?

This happens when there is a blank space after / / or nothing else. As you can see in https://www.mediawiki.org/wiki/User:Metr%C3%B3nomo. The 7th example is the most interesting.

Ok, now I can see why it behaves as external link, but still sometimes is broken and is no internal or external ...

You can see more in https://www.mediawiki.org/wiki/User:Metr%C3%B3nomo. There are two options: [ [ / / ] ] redirecting to a subpage or a page in the mainspace. Since I think that should not have sub-pages as https://www.mediawiki.org/wiki/User:Mattflaschen_(WMF)/Sandbox//Bar, I prefer the latter. But you know what is right.

That links like [ [ / / ] ] link to pages in the mainspace, now they do not link to anything.

I think that the cause is this gadget: https://es.wikipedia.org/w/index.php?title=MediaWiki:Gadget-DetectaDesambiguaciones.js, based on https://de.wikipedia.org/w/index.php?title=MediaWiki:Gadget-bkl-check.js. It is very popular in Wikipedia in Spanish, but is not enabled by default. So in that community many users perceive the problem, but outsiders are all well. Today I cleared my cache, but still did not work JavaScript. I opened the console of errors and then pointed to this script. I turned it off and cleaned my cache again, and JavaScrip worked well.

Please read https://es.wikipedia.org/wiki/Wikipedia:Caf%C3%A9/Archivo/T%C3%A9cnica/Actual#Problemas_de_interfaz