This thread shows the same/similar problem:
And it correctly identifies Dropbox as a possible culprit. However, I have a co-worker is experiencing this frequently while working on a non-Dropbox folder. He’ll commonly see the error: “All compiler errors have to be fixed before you can enter playmode”, but the error listed is a warning. There is no actual error in the code.
To “fix” the compiler error, you just have to change a random character (like add a space) and switch back to Unity to get a quick re-compile.
This problem seems to happen more frequently when you quickly change from Visual Studio to Unity and try to run the game right after changing script/code.
Even if this is a legitimate issue (the DLL file is locked by another process), perhaps it can be detected correctly and attempt to compile once the file is no longer locked? Would be much better than the current behavior.
Thanks,
Luke