It seems someone at Kraken knows about the problem, if this article is right :
http://cryptomining-blog.com/1746-possible-issues-with-kraken-withdrawals-around-the-17th-of-march/

I ordered my withdrawal on March 17th, right in the timeframe pinned in this report.

I was trading at Bitstamp before that, and gave Kraken a go for the good reviews we can read everywhere. Bad choice it seems.
I might however give this exchange a second chance, if this issue is solved quickly.

For the moment I'm still waiting for my ticket to be answered.