Old container suddenly not assigned IP on restart

I’ve got an LXD container I’ve been running on Ubuntu 20.04 (LXD version 4.0.8) as a Python/Jupyter Lab server for many months without trouble. Twice in the past couple of weeks Jupyter throws me this complaint:

File Save Error for .ipynb Unexpected error while saving file: .ipynb unable to open database file

And, in fact, it’s not making it up: I really can’t save any files.
Restarting the container doesn’t work until I reboot the host. But even then, while the container is running, it doesn’t get an IP address. The first time this happened, everything went back to normal when I rebooted the host a second time.
But is there a configuration file that I could reset manually so I don’t need to reboot multiple times? And what if a second reboot just doesn’t work next time?
When rebooting this past time, I found that the container did get its IP address, but only after around five minutes of waiting. I have no idea what was holding things up.