There are a few places that this may be an issue:
1. /usr/share/mythtv/mysql.txt which is symlinked to /etc/mythtv/mysql.txt
2. I have found that the best combination of information is to use dcerouter as the DBHostName and not localhost or the IP address
3. The last thing that sometimes cause problems is that the user is not allowed access to the database.
Try the following commands and see if you can log into the database.
$ cat /etc/mythtv/mysql.txt
$ mysql -umythtv -p(password from the above output) -h dcerouter mythconverg
if you can log on to the database then the information in the mysql.txt file is accurate. If you can not then you will need to try and few things.
if access is denied for mythtv user then try
$ mysql -uroot -p -h dcerouter mythconverg
If you can log in as root then you can change the password for mythtv user.
$ mysql> update user set Password=PASSWORD('new-value') WHERE User='mythtv';
$ mysql> flush privilege;
now change the password in the /etc/mythtv/mysql.txt file
and retry the mysql command above for the mythtv user.
If you are still denied access then you may need to allow additional access. THis is from the mythtv documentation.
$ mysql -u root mythconverg
$ mysql> grant all on mythconverg.* to mythtv@"192.168.80.%" identified by "mythtv";
$ mysql> flush privileges;
Note that the above command you should put the LAN you are using 192.168.?.%
These are the things that I have had to deal with in the past when trying to get Pluto up and running. I am not yet running LinuxMCE but I will be when my equipment arrives.
I hope this helps.