If your ETS does not start, then it could be that the 'master data' file is missing/or it is corrupt.
How to identify?
The following dialog is shown.
How to solve?
Go to https://update.knx.org/data/XML/project-21/knx_master.xml and download this file (right click on the address and then select 'Save as') to 'C:\ProgramData\Knx\XML\project-21'.
In case you cannot find this folder, it may be because it is hidden. See here to read how to make it visible.
Then, start ETS again.