Of course, an alternative would be to download the folder as compressed archive via web front-end from one server, unzip it locally and upload the contents to the other server via web front-end again. However, the upload would take much longer this way.
Is it possible or are there alternative approaches?
Copying files from one server to another is, in a first step, independent from Nextcloud, because it happens on the file system layer. Nextcloud supports to update its database by execution the "files:scan [options] [--] [<user_id>...]" command.
So the right way to go is
copy the files to the new server
adjust the directory and file access rights
execute the occ command to update the Nextcloud database
Thanks for your quick response! What about the decryption/encryption of the files? How can the files be decrypted and re-encrypted with the new key on the target server in way compatible with Nextcloud?