The option to specify LineEnd was added to 1.0.27 as a response to JENKINS-6074.
When I upgraded the plugin and started Hudson, I immediately got a number of build errors because the plugin set the value for LineEnd to the string "null", which Perforce itself wouldn't accept.
I've downgraded to 1.0.26.
I suggest that the default, if LineEnd is not present in the configuration, should be either:

• Don't specify LineEnd in the client spec and let Perforce default it
• Default to "local", which is the Perforce default, AFAIK