Hi,
4.5.1 normally isn't required for NCrunch. A change in V2 to add support for this version of the framework introduced a bug that created this problem. The link above includes a reference to a build of NCrunch that you can install that will use a v4.5 task runner that does not require v4.5.1 installed.
V2.5 of NCrunch is being officially released today which also includes the above fix. Sorry for the trouble.
Cheers,
Remco