USER Abend 2048 with the Message: "*** ERROR *** I/O ERROR READING BLOCK 1 OF MASTER FILE" when trying to open a Librarian Masterfile.
This error means that the masterfile is corrupt.
This may happen, when
If Block1 of the Masterfile is really corrupt there's no known way to recover the contents.
In this case, try to run a backup of this particular masterfile. Librarian give you meaningfull error messages for the whole MF or for particular member which is corrupt.
If the MF is corrupt, you will have to INIT a new Masterfile and run a full restore from previous backup.