Service Temporarily Unavailable

I get this Service Temporarily Unavailable. But only when I try to add a client, everything else seems to work as expected.
Any idea about cause?
Its a fresh install, latest version


Anders Yuran

Do I really have to increase the font size or make it bright red?

This is the second line
he server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

It also appears when adding payment, edit user

Why rude? Point at the solution instead. I have read faq and wiki