I have a very unusual problem which I would really appreciate some help
with, if anyone has come across something similar before.

Essentially we have three servers, a web server and two database
servers.  There are several applications running on the web server and
the databases for each are distributed across the two servers.  To even
the load it is necessary to move the database for one of the
applications to the other database server.

So,

Database Server A currently hosts the database
Database Server B is where the database should be moved to

The following steps were taken:

1) Detach database on Server A
2) Copy database MDF and LDF files to Server B
3) Re-Attach database on Server A in case rollback is required
3) Attach database on Server B
4) Change application to point at Server B
5) Test application - everything seems fine - can see data appearing in
Server B
6) Detach database from Server A

** The web application is now no longer able to connect to Server B. **

The following steps were then taken:

1) Reboot all three servers
2) Check that the connection string is definitely set in only one place
in the application

The web application is still unable to connect to Server B.

In a further peculiarity, as well as the Web Application there is also
a Windows service which accesses the same database.  This is able to
connect to Server B fine, with or without the database on Server A
being attached.

Many thanks in advance

Lewis