See the following thread on reddit:
https://www.reddit.com/r/Bitcoin/comments/72j7ur/kraken_no_longer_allowing_usd_withdraw_for_usa/Where the person having trouble withdrawing received the following message from Kraken:
Thank you very much for reaching out to us. Unfortunately, due to a temporary technical difficulty with USD funding for US residents, we will be unable to address your issue for an undetermined period of time. All other site functions are working as intended. Please let me know if there is any other assistance you need. We will update you as soon as this current issue is resolved.
That sounds like they are having problems with their American banks. Their euro withdrawals are working (they go through Fiidor bank).
