Same here, On a top of all other previously mentioned tricks I also tried to delete ~/Library/Caches/org.swift.swiftpm as well, but nothing happens. It starts happening when I switch to different branch in git and only way to make it work is to reinstall Xcode.

I found solution! It seam that SourceTree update introduce new line in .gitconfig file so getting rid if it makes this problem disappear: https://jira.atlassian.com/browse/SRCTREE-8176 Just delete [safe] bareRepository = explicit from ~/.gitconfig And SMP will start working again!