Getting a weird error when compiling C++ (Unexpected PDB error: OK (0) '?')

First question here, not sure if this is the right section but it’s C++ so it may be. Anywho - I’m getting an odd error when I try to Build my project in VS 2015 (Compile button isn’t appearing in UE so I’m using the Build option in VS instead). The log is below - I’m not sure what’s causing the issue and I can’t find anything relevant on Google. Looks like a linking error (from the LNK1318), but it may not be.

1>------ Build started: Project: ReinaU1, Configuration: Development_Editor x64 ------
1>  Creating makefile for hot reloading ReinaU1Editor (working set of source files changed)
1>  Compiling game modules for hot reload
1>  Parsing headers for ReinaU1Editor
1>    Running UnrealHeaderTool "C:\Users\Sam\Documents\Unreal Projects\ReinaU1\ReinaU1.uproject" "C:\Users\Sam\Documents\Unreal Projects\ReinaU1\Intermediate\Build\Win64\ReinaU1Editor\Development\ReinaU1Editor.uhtmanifest" -LogCmds="loginit warning, logexit warning, logdatabase error" -Unattended -WarningsAsErrors -installed
1>  Reflection code generated for ReinaU1Editor in 10.2285555 seconds
1>  Performing 2 actions (2 in parallel)
1>  ReinaU1Character.cpp
1>  [2/2] Link UE4Editor-ReinaU1-9929.dll
1>     Creating library C:\Users\Sam\Documents\Unreal Projects\ReinaU1\Intermediate\Build\Win64\UE4Editor\Development\UE4Editor-ReinaU1-9929.lib and object C:\Users\Sam\Documents\Unreal Projects\ReinaU1\Intermediate\Build\Win64\UE4Editor\Development\UE4Editor-ReinaU1-9929.exp
1>ReinaU1GameMode.cpp.obj : fatal error LNK1318: Unexpected PDB error; OK (0) '???'
1>ERROR : UBT error : Failed to produce item: C:\Users\Sam\Documents\Unreal Projects\ReinaU1\Binaries\Win64\UE4Editor-ReinaU1-9929.dll
1>  Total build time: 69.50 seconds
1>C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V140\Microsoft.MakeFile.Targets(41,5): error MSB3075: The command ""C:\Program Files (x86)\Epic Games\4.12\Engine\Build\BatchFiles\Build.bat" ReinaU1Editor Win64 Development "C:\Users\Sam\Documents\Unreal Projects\ReinaU1\ReinaU1.uproject" -waitmutex" exited with code 5. Please verify that you have sufficient rights to run this command.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Thanks!

Edit: Tried cleaning the project and got this error:
1>EXEC : error : System.UnauthorizedAccessException: Access to the path ‘C:\Users\Sam\Documents\Unreal Projects\ReinaU1\Binaries\Win64\UE4Editor-ReinaU1.dll’ is denied.
Attempting to delete it shows that it’s open in Unreal Engine.

Restarted Unreal, the compile button appeared again and now it builds. I’ll leave this in case someone else has this issue but it seems to be fixed for now.