Do I have to comfigure the room after the initial <presence/> ?
Yup according to the MUC spec.
What would be the minimum config transaction?
What you can do is either configure the room, or tell the server to use a default config by telling it to setup an instant room instead, have a look at the MUC JEP section 9.1.2 and 9.1.3.
Any idea why it works usually and stays locked only in few cases. If a roomm is locked then until server restart. There is no room config/state written to the spool folder.
The room wont be locked the first time you try to enter as you are creating it and it will wait for you to configure it, then any subsequant attempts to enter the room before its configured will be rejected with the 404.
Richard
