Xcode is unable to resolve all dependencies added to project through SPM.
One specific spm dependency is causing trouble, so when it is beingf resolved it suddenly stops with error:
the package at '/' cannot be accessed (Couldn’t read 'x.x.x': fatal: not a tree object)
I was finally able to fetch the version of spm I was after.
Difficult to say what made the trick, as the list of things tried has grown
A combination of removing some of:
reset package caches
Xcode Project Build Caches and Indexes
My guess is my machine was in a weird state in which xcode was told that my som spm was cached
But then it tried to get it and it was not there