Can't create database on volume mounted as folder (Error Msg 5123 & 1802)
Creating SQL Server Data files at the root of a volume is problematic, due to permissions issues. I always recommend creating a folder on the root of the volume, then placing data files inside that folder (this is also Microsoft's recommendation). Setting &amp; verifying permissions at the root of the drive does not always work the same as setting permissions at the folder level.

When mounting the volume as a lettered drive, this means your data files would never go directly in D:\ but instead in D:\SomeFolder\. In your case, you are using a mount point where UserDb is the root of the volume. I'd suggest creating an additional folder inside your mount point: D:\...\Data\UserDb\SomeFolder\. Then, set permissions to make sure the SQL Server service account has full control for the SomeFolder directory.

Note that this guidance from Microsoft is still true: 


  Do not install SQL Server to the root directory of a mount point, always specify a subdirectory for all files. This has to do with how permissions are granted. If you must put files in the root of the mount point you must manually manage the ACLs/permissions.


And also this more explicit advice for database files specifically:


  Warning: SQL Server does not support use of mount volume / mount point root directories for SQL Server databases.


Placing files directly in the root of the mount point means you'll have to manage permissions directly on every file to ensure they are correct. In the case of creating a new database (either from backup or from model), this can be problematic. As a result, it is best practice to never put files in the root of the mount point or drive. You can make it work, but it's not supported and not a good idea.