I saw this problem when I used the Alpha channel, so I switched back to the Release channel.
Today I installed the latest version of the XF for VS (126.96.36.1993) and Mac and I have been start getting that error again.
What I tried to fix that (non of this is working perfectly):
1. Clean & Rebild the solution
Sometime I get error like that -
Or some problems with connection to the Mac - https://bdfss.blob.core.windows.net/shots/2017-05-11_13-33-52-d11c3dbd-39b2-4f65-92a9-70a5ddc0924f.txt
2. kill all the Msbuild processes via 'tskill msbuild'
3. delete bin and obj folders manually (I never saw there locked files)
4. run VS as Administrator
5. restart VS and Mac
In general 1&2 could help me, but sometimes only the 5 can affect on the build process.
Really hard to do something with the project now. I have to spend a lot of time just for run the App.