Ha! I found a solution. Seems like my openHAB2 needs other parameters.
Disable Thing:
sendHttpPutRequest("http://<ip>:8080/rest/things/<thinguid>/enable", "application/json", 'enabled')
Enable Thing:
sendHttpPutRequest("http://<ip>:8080/rest/things/<thinguid>/enable", "application/json", 'true')
Thanks for your help!