Yes you are correct the _source field was disabled in the apm-server config and that solved my whole problem!
Don't even know why i commented this line in...
Suggestion to prevent such a thing from happening again:
There was no hint in the file that the _source field is needed for the function of the whole APM-UI, so please add a hint in the config that this will kill the whole apm functionality (because the config option is ther only commented out and with a generic hint the the es docu) or remove the option from the template config.