The problem is in the function with prototype

public boolean set(int sequence, String message)

The mistaken code says:

update.setInt(offset++, sequence);
update.setString(offset, message);

But ought to be:

update.setString(offset++, message);
update.setInt(offset, sequence);

instead.

If we see the place where INSERT_UPDATE_MESSAGE is defined

INSERT_UPDATE_MESSAGE = "UPDATE " + messageTableName + " SET message=? " + "WHERE "
+ idWhereClause + " and msgseqnum=?";

it is clear that the message ought to be stored before the sequence number.