Go to "SteamsteamappscommonDungeon Defenders 2"
Shift + right-click the folder. Make sure no other filers or folders are selected.
Select "Open command window here."
On the command window, type in "DunDefLauncher.exe --debug" without the quotes.
It will generate a file called "log.txt" in your Dungeon Defenders 2 folder.
Send that file to support[at]trendyent[dot]com with the subject "Launcher Log File."