For me it was down to a locking/permissions bug on
(path to Eclipse IDE)\configuration\org.eclipse.osgi.manager.fileTableLock
See here
Spring Tool Suite 4 (64 bit for Windows Server 2016)
Version: 4.2.2.RELEASE
Build Id: 201905232009
based on Eclipse
Version: 2.2.500.v20190307-0500
Build id: I20190307-0500
wouldn't launch and a pop up dialog appeared saying:
launch error has occurred see log file null
(This became apparent from the latest text log file in the folder
(path to Eclipse IDE)\configuration)
!ENTRY org.eclipse.osgi 4 0
2019-06-19 18:41:10.408
!MESSAGE Error reading configuration: C:\opt\sts-4.2.2.RELEASE\configuration\org.eclipse.osgi\.manager\.fileTableLock (Access is denied)
!STACK 0
java.io.FileNotFoundException: C:\opt\sts-4.2.2.RELEASE\configuration\org.eclipse.osgi\.manager\.fileTableLock (Access is denied)
...
I had to go and tweak the permissions via File Explorer (Full access).
It appeared as if the IDE was doing nothing for a while.
The splash screen for Spring Tool Suite (based on Eclipse) eventually disappeared and the IDE started up again.
Now everything is back working correctly again.