So yeah, I have encountered this numerous times in my time with UE4 (though it has gotten better). I am not 100% sure of the cause but I sort of blame hot reload on this. Essentually the DLLs are not reloading properly and/or something has locked them and is using them (Like a hanging instance of your editor).
First, check your task manager for any instance of the Unreal Edtior running in the background. Kill it and that should do it.
Second, if that doesn’t work try to perform a full Rebuild and hopefully that fixes it.
Third, Nuclear option. Delete ALL intermediate folders and binary folders from your project, this includes plugins you may have in your project’s directory. Regenerate your Visual Studio project files. And build it again. That has always worked for me, but it may also take the longest time.