Recently I ported my mythtv system from one box to another.

After getting everything set up, I noticed that mythtvbackend would
from time to time print several copies of the following error message:

ERROR: Master backend tried to connect back to itself!

I also noticed that mythtv-status reported some error messages:

Encoders:
oldhost (5) - -1 (Disconnected)
oldhost (7) - -1 (Disconnected)
newhost (11) - Idle
newhost (12) - Idle

If you google for instructions on how to solve these problems, you
will find several pages that suggest using mythtv-setup to delete all
your capture cards and redefine them from scratch.

I do not recommend that approach, because it would entail quite a lot
of unnecessary work.

Here is the smart way to do it:

1) First, stop the backend and make a backup of the existing
mythconverg database:

mysqldump -pMyPassword mythconverg | gzip > today-ok.sql.gz

It is better to have the backup and not need it than to need it and
not have it.

2) Fire up mysql and take a look at what is going on:

select cardid,hostname from capturecard;
+--------+----------+
| cardid | hostname |
+--------+----------+
|      5 | oldhost  |
|      7 | oldhost  |
|     12 | newhost  |
|     11 | newhost  |
+--------+----------+
4 rows in set (0.00 sec)

Make a note of the "cardid" values that correspond to the old host.
This should agree with what mythtv-status was telling you; see above.

3) You want to delete the records that refer to the old host.  In the
following sql commands, change "5" and "7" to the values appropriate
to your old host:

delete from capturecard where cardid=5;
delete from capturecard where cardid=7;

There are other tables that require the same treatment:

delete from cardinput where cardid=5;
delete from cardinput where cardid=7;

delete from channelscan where cardid=5;
delete from channelscan where cardid=7;

If there are no records in the channelscan table don't
worry about it.

4) Restart the backend.  Test by watching some live TV

That's all there is to it.