● acme-spam.lossy.network.service - Renew ACME certificate for spam.lossy.network
     Loaded: loaded (/etc/systemd/system/acme-spam.lossy.network.service; enabled; preset: ignored)
     Active: activating (start-post) since Mon 2024-11-25 12:40:50 UTC; 3 days ago
 Invocation: de04e9b6744344cf86391a79578da590
TriggeredBy: ● acme-spam.lossy.network.timer
    Process: 180885 ExecStart=/nix/store/h9h8x07dqvzp9652p05jq8zs2205q66v-unit-script-acme-spam.lossy.network-start/bin/acme-spam.lossy.network-start (code=exited, status=0/SUCCESS)
   Main PID: 180885 (code=exited, status=0/SUCCESS); Control PID: 180907 (vw98jrrshrz371a)
         IP: 23.3K in, 12.4K out
         IO: 52.1M read, 8K written
      Tasks: 2 (limit: 4553)
     Memory: 1.8M (peak: 105M)
        CPU: 577ms
     CGroup: /system.slice/acme-spam.lossy.network.service
             ├─180907 /nix/store/0irlcqx2n3qm6b1pc9rsd2i8qpvcccaj-bash-5.2p37/bin/bash /nix/store/vw98jrrshrz371az32ssbcwrr3bz2fqs-acme-postrun
             └─180909 systemctl reload nginx

Nov 25 12:41:01 helios acme-spam.lossy.network-start[180900]: 'certificates/spam.lossy.network.crt' -> 'out/fullchain.pem'
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: + cp -vp certificates/spam.lossy.network.key out/key.pem
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180901]: 'certificates/spam.lossy.network.key' -> 'out/key.pem'
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: + cp -vp certificates/spam.lossy.network.issuer.crt out/chain.pem
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180902]: 'certificates/spam.lossy.network.issuer.crt' -> 'out/chain.pem'
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: + ln -sf fullchain.pem out/cert.pem
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: + cat out/key.pem out/fullchain.pem
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: + chmod 640 out/cert.pem out/chain.pem out/fullchain.pem out/full.pem out/key.pem out/renewed
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: + echo 'Releasing lock /run/acme/2.lock'
Nov 25 12:41:01 helios acme-spam.lossy.network-start[180885]: Releasing lock /run/acme/2.lock

In reply to @hexa:lossy.network
m1cr0man: https://gist.githubusercontent.com/mweinelt/b27e2353eedc99242a1074a5d2a4e85f/raw/2475ed5c4863f763f8e0e3938dcc2567a633d67d/gistfile1.txt

It's not stuck on "Releasing lock", that process has exited: Main PID: 180885 (code=exited, status=0\/SUCCESS)
The code actually does nothing after printing that, the script just exits which automatically frees the lock (source)

Based on the CGroup content I think it's stuck on reloading Nginx though I don't understand how that would block or why it's doing that, as Nginx is supposed to reload itself through nginx-config-reload.service.
What's the content of /nix/store/vw98jrrshrz371az32ssbcwrr3bz2fqs-acme-postrun?
What's the value of services.nginx.enableReload? Did you add nginx to the cert's reloadServices?

#!/nix/store/0irlcqx2n3qm6b1pc9rsd2i8qpvcccaj-bash-5.2p37/bin/bash
cd /var/lib/acme/spam.lossy.network
if [ -e renewed ]; then
  rm renewed
  systemctl reload nginx

  
fi

#!/nix/store/0irlcqx2n3qm6b1pc9rsd2i8qpvcccaj-bash-5.2p37/bin/bash
cd /var/lib/acme/spam.lossy.network
if [ -e renewed ]; then
  rm renewed
  systemctl reload nginx # <-- stuck here
fi

  security.acme.certs."spam.${config.networking.domain}" = {
    postRun = ''
      systemctl reload nginx
    '';
  };

Thread 1 (Thread 0x7f7f4c1c5680 (LWP 180909) "systemctl"):
#0  0x00007f7f4c50963c in ppoll () from target:/nix/store/pacbfvpzqz2mksby36awvbcn051zcji3-glibc-2.40-36/lib/libc.so.6
No symbol table info available.
#1  0x00007f7f4c82270b in ppoll_usec () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#2  0x00007f7f4c89e33a in bus_poll () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#3  0x00007f7f4c89e6c5 in sd_bus_wait () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#4  0x00007f7f4c6c41b9 in bus_wait_for_jobs () from target:/nix/store/ivqjhj99firnjq7gp14qf35821viwi5m-systemd-256.7/lib/systemd/libsystemd-shared-256.so
No symbol table info available.
#5  0x00005641d8690e2c in verb_start ()
No symbol table info available.
#6  0x00005641d8672bea in main ()