This is what I could find in Soft 968 Firmware Guide.
I have only seen read error d if I cat a file saved with "amsback"
c. Error messages.
While searching for a block of the file being read, a higher numbered
block than that required has been found. The required block has been
missed. This message is often produced after a read error in the
required block when the next block is found.
Read error X
An error of some kind occurred whilst reading from tape. The tape
should be rewound and the block played again. The X is a single
letter indicating what kind of read error occurred:
'a' Bit too long An impossibly long one or zero
has been measured. This often
indicates reading past the end
of the record.
'b' CRC error Data was read from tape
'd' Block too long The data record contains more
than the expected 2048 bytes of
Write error a
An error occurred whilst writing to the tape. There is only on
possible write error. This indicates that the Cassette Manager
was unable to write a bit as fast as was requested. This error
will never occur unless the user has set the write speed beyond
the maximum possible.