There are some times where you encounter an issue while using Plastic and Unreal, but the log window in Unreal is not showing anything but a generic error message and there is no clue what might be happening in Plastic's logs.
When you get to that situation, is time to call for the plugin logs.
Enable UTC Timestamp:
In order to make those logs easier to understand it would be really helpful to activate the UTC Timestamp. To do it, go to Unreal and select "Preferences" --> "General" --> "Appearance" --> "Log Timestamp mode".
Keep in mind to do this if you report the issue in the proper Github project too.
You can find these logs in the project folder, go to "ProjectName/Saved/Logs/" and then pick up the "ProjectName.log" file you should encounter.
And that is all! Sending those logs will be very helpful when debugging issues.