Weird behaviour / "unknown error" when entering wrong password with CUPS

When using the CUPS webinterface (http://localhost:631/) via QtWebKit and entering a wrong password (e.g. via Administration -> Add printer), weird things happen:

• With Firefox, I get another dialog asking me to authenticate, as expected.
• With the example browser, cups uses 100% CPU and nothing happens - this might be a CUPS bug, but I can't trigger it with any other browser
• With the Otter browser, I get an error page saying "Unknown error".