I would like to be able to validate bpmn diagrams. There seems to be a switch for diagram validation that is switched off by default: https://app.camunda.com/jira/browse/CAM-1983. Where can I find this switch? Or is validation of a diagram not possible?
Answer
The diagrams you model are structurally valid by definition. Read the detailed answer below.
What exactly would you like to do?
Designtime. So on press of the button “download diagram” get error-info. It’s about BPMN 2.0 XML compliance. So if I have f.e. an endevent only with nothing else in a diagram, or only a servicetask with no in- or output, design-time, I would like a user to get some info when he tries to download the xml.
You’d like to validate whether the process is executable by a process engine?
A service Task alone is valid BPMN (at least inside sub-processes).
Excuse me, I don’t know all bpmn2.0 rules. But no, the only functionality that I would like to have client-side concerning validation is compliance to bpmn2.0. Nothing vendor-specific, althought it would be nice to be able to add vendor-specific rules in a userfriendly way yourself. I thought, based on the link in my first mail, that this maybe was implemented already. A special button for diagram validation would be nice, so a save/download button without validation - also for saving work without validation messages while the diagram is not complete yet - and a validate button to see if the diagram is bpmn2.0 compliant.
Our libraries and the tools built upon them are quite different from most others you see in the market: We disallow “invalid” constructs to be modeled any time. And because the tools are backed up by the respective standards meta-model we are very sure about what is allowed and what is not.
Because of that, whatever you export will always be valid BPMN 2.0 (everything else is a bug that needs to be fixed). So technically, you do not need the validation button you mention, even though you may be used to it from other modeling tools 
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Disclaimer: Obviously there is a lot of things you can validate on top of structural BPMN 2.0 compliantness. As I understand you, that is not what you are asking for.
That is good to hear, thank you!