Troubleshooting
See: Troubleshooting: *.ini files
System.IO.IOException: The cloud file provider is not running.
If you're syncing your Documents folder with OneDrive, make sure that OneDrive is running and sync is on.
You can also disable the back up feature... (click to see how)
You can also disable the back up feature by going into OneDrive's settings → Back up → Manage back up
then unselect all three folders:
If you don't want to disable it, go to your OneDrive\Documents\My Games folder, right-click the Fallout 76 folder, and click "Always keep on this device".
(You probably need to navigate to the folder starting from the OneDrive folder to have this option available.)
See: Troubleshooting: DPI Scaling issues
If you're running the mod manager with administrator privileges, Windows will block drag-and-drop for security reasons. (Read more in this blog post)
If possible, try running the mod manager without admin rights. If you can't, because the tool will otherwise crash, use the import buttons/links. They should always work.
I noticed that for some Xbox Game Pass users, the tool just crashes when it tries to write to the game folder.
Running the tool as admin should normally fix this. But this comes with some caveats. (See I cannot drag-and-drop files into the mod manager)
Look into "archive2.log.txt". If you can spot "System.IO.FileNotFoundException: Could not load file or assembly 'Archive2Interop.dll'", then Visual C++ Redistributable for Visual Studio 2012 Update 4 is missing. Download and install it.
I've found the solution here, btw: https://www.nexusmods.com/fallout4/mods/28795/?tab=posts
EDIT: From version 1.8 onward, the tool will display an error message, saying "Archive2 failed to pack/extract archive".