Just wanted to document a weird error I’d been having for the past day where the WordPress theme selector would give the following error when selecting a Sage-based theme: "/current/web/app/themes/mytheme is not readable ".
No amount of chmod -R 777 would help me and I was at a loss how to proceed.
When I thought to try deleting a hidden folder I assume was created by yarn install called “.cache-loader” in the Sage theme root.
This ended up being a simple fix. Hope no-one has to lose time over this so posting the solve here.