i had a look on that doc page before submitting.
Unfortunally the journalctl command does not give many more information:
Nov 22 00:03:46 cCaddy systemd: Starting Caddy...
Nov 22 00:03:46 cCaddy systemd: caddy.service: Failed to execute /usr/bin/caddy: Resource temporarily unavailable
Nov 22 00:03:46 cCaddy systemd: caddy.service: Failed at step EXEC spawning /usr/bin/caddy: Resource temporarily unavailable
Nov 22 00:03:46 cCaddy systemd: caddy.service: Main process exited, code=exited, status=203/EXEC
Nov 22 00:03:46 cCaddy systemd: caddy.service: Failed with result 'exit-code'.
Nov 22 00:03:46 cCaddy systemd: Failed to start Caddy.
Tried it out with a brand new container with debian 11.1 and directly after apt install caddy, even within the apt install output, the system outputs the same error.
Seems to be a problem regarding the 5.13.19-1-pve kernel, error is occurring since the update.
Gonna have a look into it, maybe I can find a workaround etc.
May be a problem caddy might facing with upcoming kernel updates.
I tested everything today and booted the system with the 5.11.22-7(-pve) kernel instead of the ?5.13.19-1(-pve) version. Everything is starting up there with no problems.
So it seems to be a problem occuring with the newest linux kernel versions, and it seems to be either a problem with go not being able to create new procresses within the service, or the caddy.service configuration not applicable with 5.13.19-1(-pve) or further.
I hope that might be interesting for the dev team.
We use the current proxmox release 7.1-6 with a debian 11.1 container.
Hopefully i could help you a bit and this might be interesting.