I guess something must have changed on the build agents (and you've gotta love Windows).

E.g.

http://teamcity.codebetter.com/viewLog.html?tab=buildLog&buildTypeId=Scriptcs_CI&buildId=204147

[01:14:25][ScriptCs.Tests.Acceptance.dll] ScriptCs.Tests.Acceptance.Packages.PackageContainsAFrameworkAssemblyReference() [01.01.02] When I install a package which contains a framework assembly reference (1s)
[01:14:26]
[ScriptCs.Tests.Acceptance.Packages.PackageContainsAFrameworkAssemblyReference() [01.01.02] When I install a package which contains a framework assembly reference] ScriptCs.Tests.Acceptance.Support.ScriptCsException : scriptcs.exe exited with code 1. The output was: Installing packages...
ERROR: Error installing package. [PathTooLongException] The specified path, file name, or both are too long. The fully qualified file name must be less than 260 characters, and the directory name must be less than 248 characters.