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:".
Also make sure no antivirus is preventing the correct functioning of the Launcher.