I use KMail 1.13.5 and KDE 4.5.0. I manage several mail accounts with KMail (gmail / freenet). Since a few days everytime i want to send an email i recieve an error message:

SASL(-4): no mechanism available: No worthy mechs found

Counts for all accounts, gmail and freenet. No account settings was changed, the same settings does work in thunderbird.

What is going wrong? Does anyone have the same problem?

Have you tried recreating your SMTP server?

I added a new smtp profile. But this doesn't work, too.

Re-check your mail supplier's smtp requirements. Something like this happened to me once. It turned out that my ISP had made changes. For some inexplicable reason, as long as I used my system without a change, the old settings worked, but in the new installation I had to use the new settings. It sounds illogical, but that's exactly the truth. It may have been coincidence, or maybe I was no longer on the same server - I've no idea.

It works with Thunderbird, so it can't be a problem of the ISP.
It occures with gmail and freenet accounts, both requirements are checked and fulfilled.