Today I had a strange error message.
Notes error: Unable to open Name and Address Book (%s), due to error '%e'. Locations cannot be used until the problem has been corrected.

The user had changed nothing regarding the NAB or his locations. Neither had I. We both got this error message.

After a while we figured out that the error was caused by an administrator, who had restored a database from the backup into the Domino data directory. So there were two databases with the same replica id. Which is not a good idea. So the administrator deleted the older database. And then things started to get complicated.
The error was thrown by an LotusScript agent because he tried to open this database by replica id. When the Domino server searched for the database the internal table entry for this replica id pointed to the deleted database.

Afterwards I could understand what caused the error. But the error text was not much of a help. I guess the Notes client couldn't find any better error message and came up with this one.

P.S. We tried to get rid of the wrong entry. But "dbcache flush all" didn't worked. This night the administrator, who restored the database from the backup, will restart the server.