Argh! After posting this I found the issue.. Basically, the project had dependencies on other projects. The csproj files for those related projects were slightly wrong – and had this as the SolutionDir:
<SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\..\CableSense\</SolutionDir>
Which then caused the issue! Hope this helps someone else there, I feel bad for answering my own question now.