It seems not to be possible to save language links with an Umlaut. Try for example to link to Ästhetik on the German language Wikipedia.
The link is not saved and the following error displayed: "A connection error has occurred while trying to perform save, and because of this your changes could not be completed. Please check your Internet connection."
Version: master
Severity: critical
Also Cyrillic, Greek, anything non-ASCII apparently. Smells like bad encoding settings somewhere.
I don't have this problem with my own test repos. Only happens on the test wiki: http://wikidata-test-repo.wikimedia.de/
There were no changes made on encoding settings but I got an error message from apache and set a higher memory limit for php. Tried adding language links with German Umlaut and Cyrillic characters which now worked for me.
Interesting... appears this is related to https://bugzilla.wikimedia.org/show_bug.cgi?id=40804 about memory leak
It still worries me that we need so much memory for processing this stuff. Something appears to be wrong here...
Changing from Blocker to Critical as upping the memory limit allows to link to Umlauts again, but needs investigation in why it costs so much memory.
It is now possible to store a link to Ästhetik on the German language Wikipedia in the master branch.