I add a NuGet package (library) called EPPlus (doesn't really matter I guess) to export sheets to excel and right after that program for some reason at some random point during runtime gives an error saying some random source fine not found. Why in the world is this happening? Solution was built well. I tried experimenting with "Just my code", checking/unchecking boxes "NuGet Symbol Servers" and "Microsoft Symbol Servers", tried decompile source to symbol file that made no difference, to tried installing different versions of that library. What else is there to try? I guess it has to do with adding some symbols but it is quite complicated to comprehend this concept for a new coder. Please, let me know how to fix it?