Cannot build for Facebook Gameroom

This is what I get when I try to build for Facebook Gameroom:

UnityEditor.BuildPlayerWindow+BuildMethodException: Build failed with errors.
at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (BuildPlayerOptions options) [0x001b9] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:162
at UnityEditor.BuildPlayerWindow.CallBuildMethods (Boolean askForBuildLocation, BuildOptions defaultBuildOptions) [0x00050] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:83
UnityEditor.HostView:OnGUI()

 

I'm not sure, but I think it was the only error message I saw, the rest were just warnings... But... There's something funny here... I opened the project with Unity 5.6.f3 and built the player, then I opened it up again with Unity 2017.1.0b6 and it now I can build the player... it seems like there's an import problem when you upgrade from Unity.1.0b5... because going back to a previos version and returning seemed to solve the problem.

However I get the following warnings:

Timestamps (800) and assets (758) maps out of sync - repairing...
UnityEditor.HostView:OnGUI()

The AssetDatabase contains an asset with GUID '67e9dad5654047ebbe623cce9dbf7b38', but an empty path. This asset will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains an asset with GUID 'f48df919adea4fa095a7407e773e5aa4', but an empty path. This asset will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains an asset with GUID '307433eba81a469ab1e2084d52d1a5a2', but an empty path. This asset will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains an asset with GUID '6cdf1e5c78d14720aaadccd4c792df96', but an empty path. This asset will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains a timestamp for 'C:/Program Files/Unity/Editor/Data/UnityExtensions/Unity/TestRunner/Editor/UnityEditor.TestRunner.dll', but no mapped GUID. This timestamp will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains a timestamp for 'C:/Program Files/Unity/Editor/Data/UnityExtensions/Unity/VR/Win64/OVRPlugin.dll', but no mapped GUID. This timestamp will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains a timestamp for 'C:/Program Files/Unity/Editor/Data/UnityExtensions/Unity/VR/Win/AudioPluginMsHRTF.dll', but no mapped GUID. This timestamp will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains a timestamp for 'C:/Program Files/Unity/Editor/Data/UnityExtensions/Unity/TestRunner/portable/nunit.framework.dll', but no mapped GUID. This timestamp will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains a timestamp for 'C:/Program Files/Unity/Editor/Data/UnityExtensions/Unity/UnityHoloLens/Editor/UnityEditor.HoloLens.dll', but no mapped GUID. This timestamp will be removed from the database.
UnityEditor.HostView:OnGUI()

The AssetDatabase contains a timestamp for 'C:/Program Files/Unity/Editor/Data/UnityExtensions/Unity/UnityPurchasing/Editor/UnityEditor.Purchasing.dll', but no mapped GUID. This timestamp will be removed from the database.
UnityEditor.HostView:OnGUI()


Too many to post all here...

 

I'm not sure and I cannot reproduce the error once I opened it with Unity 5.6.f3. This makes me think that Unity 2017.1.0b5 (or previous) screwed some meta-file or internal parameter and passed wrong all the way to b6. I don't know what would happen if I go back and take the project throughout all the beta path since 0b1 to 0b6... I usually uninstall the previous beta on each upgrade so I don't have the means to perform such test. I tried to build the facebook player only until 0b6, so I cannot tell in which version the problem started. The fact is that downgrading to 5.6 and then upgrading to b6 solved the problem, why? I don't know.