If you attempt to compile a project while an editor process is running, even if it is of another project entirely, it will hot reload the project instead of doing a normal compile. This can create issues with projects that don't have their .dlls yet as the correct .dll will not be generated. The process that is open must be the same engine version for this to occur.
This also occurred in 4.13
Head over to the existing Questions & Answers thread and let us know what's up.