Netbeans recently informed me that new updates were available. I clicked the “Update” button, unsuspectingly. When Netbeans finally restarted, it only popped up briefly and then closed again immediately. A moment later the splash screen appeared; Netbeans started again, closed again, and so on, until I forcefully terminated the process.
Apparently this problem is caused by failed updates. In my case, the solution was to go to Netbeans’ installation path and emtpy the subfolder
enterprise\update\download completely. After that it started normally again, and a new attempt to install the update was then successful.