I find out what you need to do. When you click withdraw you just need to wait. You should never press F5 or reload button or else page will fail with errors like your or few similar (for example increase funding limits etc...).
While the second part of this is true - the system blocks direct links and reloads - the first part is certainly not. When you click the Withdraw link, you get that error message, and waiting does nothing. That page doesn't refresh, at least not for me. It just sits on that message forever.
I am very hopeful that, given the announcement that they are aware of this, Kraken is working on a fix today.