Xamarin - Error IDE1100 Error reading content of source file

Cleaning the project did not solve, but closing and reopening solved my problem.

In addition to the answers of E.Zhao and messyprogrames, here a step-by-step walkthrough.

Clean Solution:
Navigation Bar > "Build" > "Clean Solution"

Unload project:
Choose your "problem project" on the right side within the solution explorer. Right mouse click > "Unload Project"

Reload project:
Choose your unloaded "problem project" again. Right mouse click > "Reload Project".

Opening and closing the solution wasn't helping, but unloading and reloading the project worked.