Weird error when I compile : IBTool exited with code 255

Hi,

using Visual studio comunity version 7.4.1, on Macos high sierra 10.13.3
I got on github a sample of RSS reader : https: // github.com/Mahabali/XamarinRSSFeed

when I compile, at first : I can run it.

the IOS project target IOS 10.0
the droid project target latest android 8.1

then, all package needed to be updated.
si I updated all package,
and now, I have 2 errors, which (for me) are not telling me what is wrong :
IBTool exited with code 255
the "IBTool" task failed unexpectedly

I tried to delete all bin and obj, and I tried to clean ll before build, without success.

what can I do ?
except delete visual studio, nuget packages, and reinstall everything ? (this is not an option for me )

thanks
Olivier