See https://support.atlassian.com/browse/BSP-631

Currently Bamboo assumes that it can run the p4 users command without being logged in. Because of this the user is not actually set before it runs the command, and, when it cant run the command, it must look to the system user for login which is often not set up.

A workaround solution is to have a line in your protect file for all users (e.g. list user * * //...) or create the system root user.