You should have a log file with the incident report in (at least something close to) : /WebSphere/AppServer/profiles/default/logs/ffdc/<your server name>
.
It should contain more information (and a stacktrace).
EDIT :
Websphere 6.1 is qui old already, in fact IBM does not support it anymore since 30 sept 2013.
Since to problem seems to be known and fixed in later release, I suggest you update to a more recent version, at least to a recent fixpack of 6.1. You can learn more about the process in this page. 6.1.0.47 FixPack is the last one. You simply need to download the PAK file and run the installer.