When I added a webService to the project that implemented a connection to a ReportViewer, a build returned the error that it could no longer find the reference DayPilot Pro.
For anyone who encounters this problem, the solution lies as follows:
1. Go to Solution Properties (rt-click the Solution name and choose Properties).
2. Click Build options.
3. At the bottom, you see a dropdown beside "Generate serialization assembly". This defaults to "Auto" but changing it to "Off" solves the problem.