I have what seems to be a very strange problem with a windows forms application. I am a web developer and I have limited experience of developing windows forms application.
I just recently had an issue like this one, and wanted to include my solution for future reference in case it comes up again. There's a warning (not an error) message that pops up when the build fails, which I missed originally because I had my error list set to filter warnings out. In my particular case I needed to change away from using the client version of .NET, and the warning message contained the information I needed to debug the problem.
Warning 44 The referenced assembly [...] could not be resolved because it has a dependency on [...] which is not in the currently targeted framework [...]. Please remove references to assemblies not in the targeted framework or consider retargeting your project. [...]"
Your library possibly has dependencies on other .NET libraries such as System.Web that you need to reference in your Forms app. That can create this sort of behaviour.