Why is Mavericks forcing a password reset on every lock screen?
I figured it out. Somewhere during the upgrade process the password policy maxMinutesUntilChangePassword was set to 100. So every 100 minutes I had to reset the password...

To solve it I used the pwpolicy command:

pwpolicy -u "MyUserName" -setglobalpolicy "maxMinutesUntilChangePassword=86400"