Can you spot any pattern which jobs can be started and which not? Is there any relevant log info in Jenkins' log file?

Ok, i found the cause.
The user which is used to log in to im had'nt enough rights.

I have found attached rights image possible to set. Can u please document which rights maps which xmpp chat (!help exluded) command, so that everybody knows that this user has to have thats right to be able to execute such commands.

Tx.

I cannot see which rights you checked on the attached screenshot. Can you please attach a one showing that?

Why should you, there is nothing to see then which right are available for jenkins. As you see above myquestion was how it is implemented. Which rights map which chat command, so that the command ca be executed! If you did the code you shall attach a description how this 't works. It wasn't my intension to reverse enginere and document the plugins manual by try and error.

However i gave all rights (without admin), and then of course it works. I won't think that this is best practice.

Again please write a manual how this chat commands map to jenkins user rights.
This is a request for better documentation.

Okay, so I misunderstood your previous comment.
AFAIR you need Job#Build rights to trigger a build (and I found this pretty obvious). However, I admit that documentation is missing in big parts.

BTW: this is an open source project, so you're invited to amend the documentation in the Wiki if you find something missing.