A WebDAV network drive that disappears after a restart

The question: you've mapped your Nextcloud as a network drive in File Explorer (see Your Nextcloud as a network drive (WebDAV) in Explorer or Finder). After a restart the drive is gone, shows a red cross, or Windows keeps asking for your password.

Why: the built-in WebDAV client in Windows is sensitive. It needs a Windows service that sometimes starts too late, doesn't always remember the password, and has a limit for large files.

Check these points

1. Use an app password. Your regular password won't work, because your Nextcloud asks for a login code. Create an app password (App passwords: when and how to use them) and use that when mapping the drive.
2. Let Windows remember the password. Remove the drive (right-click, Disconnect) and map it again, ticking Reconnect at sign-in, Connect using different credentials, and then Remember my credentials. You'll find the saved details in Windows' Credential Manager, under Windows Credentials.
3. Set the WebClient service to automatic. Type services.msc in the start menu, find WebClient, open it, and set Startup type to Automatic. Click Start. You'll need administrator rights for this.
4. Use the full address with https. The address ends with /remote.php/dav/files/ and your username, with a forward slash at the end. You'll find it in Files under Files settings.
5. Has your password changed or the app password been revoked? Then Windows will keep trying the old one. Update it in Credential Manager. An old connection that keeps logging in incorrectly can cause your IP address to be throttled; see IP address throttled by an old app: which one is it?.

Files larger than 50 MB

By default, Windows won't copy files larger than about 50 MB via WebDAV. You'll get a message saying the file is too large. This limit is set in the Windows registry (the FileSizeLimitInBytes value under the WebClient service) and can be increased to up to 4 GB. This is a job for someone familiar with the registry. An easier option: use the browser or the desktop app for large files.

Does it keep disappearing?

If so, use the Nextcloud Files desktop app with virtual files (Installing Nextcloud Files on Windows and Virtual files: see everything, only download what you open). This way, you'll see all your files in File Explorer without them taking up space, and the connection is far more stable. You won't need the network drive any more after that: disconnect it and revoke the app password (Viewing, revoking, or remotely wiping devices and sessions).

Tip (Mac): on a Mac, too, a connection made via Connect to Server disappears after a restart. If you want it to come back automatically, add the connected drive under System settings, General, Login Items & Extensions, under Open at Login. Or use the desktop app here as well.