Make sure the installation path for Median XL (not the Launcher's one) matches with the installation path of Diablo2+LoD. You can check the Median XL's installation path in the setting page of the Launcher, to access click on the cog "⚙️" icon (up-right) in the Launcher window. If you do not remember where you installed Diablo2 LoD you can check its installation path by 'right-clicking' on Diablo2 LoD original shortcut, then selecting "Properties" and in the "Shortcut" tab read the path written in "Target:", the needed path should ends with the folder "Diablo II".
If the problem persist and the path is correct, open the Launcher and go into its settings page, there click on "Application Data" (bottom-right corner), the **errorLog** is in the "logs" folder inside the "Launcher" one.
Post it here.