"Access to the path "...Assembly-CSharp-firstpass.dll.mdb" is denied" error

I constantly get the error: “Access to the path “C:\Unity 5 Projects.…Assembly-CSharp-firstpass.dll.mdb” is denied”. Right now, even rebooting didn’t solve it (usually either restarting Unity or rebooting the computer fixes it, but not this time). This is literally crippling, at a time when I’m in a hurry to get a project done today. There are lots of other posts about this error all over the forums, but no fixes that actually work reliably.
How can I fix this, and preferably keep it from happening again? It really makes Unity unusable.

I finally solved it by uninstalling McAfee and then telling Malwarebytes to allow Unity access. Or at least, it’s been solved for now.