Internal error when trying to view build results

The following error occoured when trying to view the results (summary tab) of a remote agent:

Version: 5.1.0
Build: 3901
Build Date: 28 Aug 2013
Request information:

Request URL: http://baufix:8085/chain/result/viewChainResult.action
Scheme: http
Server: baufix
Port: 8085
URI: /chain/result/viewChainResult.action
Context path:
Servlet path: /chain/result/viewChainResult.action
Path info:
Query string: planKey=EMSYS-TLRTESTTOOLS&buildNumber=363

Stack Trace:

Method public com.atlassian.bamboo.v2.build.agent.BuildAgent com.atlassian.bamboo.ww2.actions.chains.ViewChainResult.getAgent(long) threw an exception when invoked on com.atlassian.bamboo.ww2.actions.chains.ViewChainResult@608c72b
The problematic instruction:
----------
==> assignment: agent=action.getAgent(jobSummary.buildAgentId)! [on line 34, column 21

Please have a look on the attachment "Error-Page".txt which includes the java dump.
This error occours sometimes, and sometimes there is no error.