Last modified: 2013-06-26 12:41:53 UTC
When you click [add] in the non-JS view to add a new sitelink, you're brought to Special:SetSiteLink but the prefilled siteid is invalid. It is e.g. "en" instead of "enwiki".
Fixed in https://gerrit.wikimedia.org/r/#/c/67862/ Besides, it happened when you've clicked [edit]. [add] doesn't set any siteid.
Verified in Wikidata demo sprint 22-9