Authorization problems - Clients can't log in, can't add mail addresses

[ranageman]

Suddenly (no particular event seems to have triggered this) I have two problems:

- Clients cant log in ("Invalid username or password, please try again.")
- Even when logged in as "admin" I cannot perform certain admin tasks on a domain.
Specifically, the "Add New Mail Name" icon is grayed out for all domains.

Otherwise the server has been running without problems for several years. I've tried restarting the whole VPS without any effect.

Help!