backup failed read-only files?

Backup has not run for several days giving folloqing error -

System.Data.SQLite.SQLiteException (0x80004005): attempt to write a readonly database
attempt to write a readonly database
   at System.Data.SQLite.SQLite3.Reset(SQLiteStatement stmt)
   at System.Data.SQLite.SQLite3.Step(SQLiteStatement stmt)
   at System.Data.SQLite.SQLiteDataReader.NextResult()
   at System.Data.SQLite.SQLiteDataReader…ctor(SQLiteCommand cmd, CommandBehavior behave)
   at System.Data.SQLite.SQLiteCommand.ExecuteReader(CommandBehavior behavior)
   at System.Data.SQLite.SQLiteCommand.ExecuteNonQuery(CommandBehavior behavior)
   at System.Data.SQLite.SQLiteCommand.ExecuteNonQuery()
   at MailClient.Storage.Data.DataStore.GetConnectionInternal(String relativeName)
   at DbBackup.FormBackup.Backup(CancellationToken cancellationToken)
   at DbBackup.FormBackup.<mainform_load>b__27_0(Object a, DoWorkEventArgs b)</mainform_load>
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)
Data:
SQLiteCommandText: pragma auto_vacuum=incremental;

What needs to be done?