1 Error message:

SQL6048N
A communication error occurred during START or STOP DATABASE MANAGER
processing.

SQL1032N No start database manager command was issued. SQLSTATE=57019

2 Possible causes:

You might get this error when starting an instance with the db2start command.
This error could happen because your db2nodes.cfg file is not correctly indicating your host machine name as one of the nodes. 

3 Investigating

Check de db2nodes.cfg file at SQLLIB directory. You should get your hostname on the file.

Example on a non-partitioned db:

user> cat db2nodes.cfg
0 <hostname> 0

4 Resolution

To fix that:

If the name shown is not the name of your host, then change it(using a vi editor for example) and try to start the instance again with db2start command.