Hi Kilian,
I faced similar issue when Mendix tried to access my db.
For this run, you might have using some port right. Try to terminate that port and again run
To terminate PORT,
Command: netstat -ano | findstr :8080
The above command with give you PID. use that PID in below command
2.Then terminate the process.
taskkill /PID 22680 /F
It errors out at the ‘exporting custom widgets’ step? What Mendix version are you using and what widgets are in the widgets folder?