It's a good idea to clean up the backup log after you purge backup media. Use a text editor to do so.
Be careful with your edits: once BACKUP or RESTORE records information in this file, it does not check its accuracy.
There is only one backup log on a server. The server must be able to read and write this file. The system administrator may want to limit access to this file by other users. If you are running more than one database server on a system, you should set the IQLOGDIR15 environment variable differently for each server, to produce separate backup logs.