SQL CE is a database that is file based with a single file that is the database and this error means that the permissions on the file are not correct. The user that is the identity on the IIS application pool needs full control over the database file. The file itself (a .sdf file) lives in the /App_Data folder so you need to find it and fix the permissions on it so that the user has full control to read and write to that file.
Note that if you download and upload the file again it will lose the file system permissions every time you do that so if you upload the file you must fix the permissions every time after uploading it.
This is the exact same problem I helped you with before on this thread and the solution is the same today as it was then.