Troubleshooting

I cannot drag-and-drop files into the mod manager.

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.

The mod manager crashes with "System.UnauthorizedAccessException" when I try to install mods.

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)

I can't run Archive2.exe / *.ba2 files are missing

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".

"You need a new app to open this bethesdanet-link" dialog when launching Fallout 76

The bethesdanet:// protocol might not be associated with the Launcher. Try to reinstall or repair the Bethesda.net Launcher.

System.UnauthorizedAccessException / Access denied: "C:\Users<yourname>\AppData\Local\Fallout 76 Quick Configuration"

Please check if you have write-permission to the folder:

Right-click on the folder, click on "Properties".
Uncheck "Read-only", if checked.
Then click on the tab "Security", select your user profile.
Check if you have all permissions. If not, click on "Edit", and tick all the boxes except "Special permissions". It should look somewhat like this (Screenshot).