Summary: Home-manager as NixOS module, sd-switch, impermanence (FUSE bind mounts), activation timeout
I’m honestly at a loss and hoping someone here has seen something similar or has an idea because I’ve run out of things to try 
My NixOS system started behaving strangely from one day to the other. During nixos-rebuild switch, the activation phase fails because Home-Manager activation times out. When rebooting, the system takes 6+ minutes because Home-Manager hangs until timing out after 5 minutes.
From the logs, it looks like it’s:
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Trying to unmount bind mounts
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Then immediately trying to remount them
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Timing out
It’s not a specific bind mount causing this - it varies each time.
Setup:
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Home-manager as NixOS module
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systemd.user.startServices = "sd-switch" -
Impermanence with FUSE bind mounts for home directories
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Using the
home-manager-v1branch of impermanence (pinned due to unrelated bug in v2)
What makes this even more confusing:
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I reviewed all changes I made before this started happening and found nothing related
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Booted into a previous generation → same issue
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Checked out an older commit that definitely worked before → still broken
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This suggests something outside my config changed
I’ve run out of ideas… any pointers on what to investigate would be greatly appreciated! If you have a good strategy to narrow it down: Please let me know! 
Repo with minimal reproduction of my config structure: https://github.com/AcidDemon/home-manager-problem