(network.c.543) SSL: couldn't read X509 certificate from '/etc/lighttpd/server.pem'

I attempted to install a Let's Encrypt certificate as shown here: https://github.com/photinus/ubnt-letsencrypt

After attempting to install, I am receiving the error mentioned in the title. I also can't access the GUI. Wondering how to either restore the router to a working state or start fresh, whichever is easier.

The server.pem file seems OK, it begins and ends like:

-----BEGIN RSA PRIVATE KEY-----
......
-----END RSA PRIVATE KEY-----

Here are the permissions:

username@ubnt:~$ ls -lha /etc/lighttpd/server.pem
-rw-r--r-- 1 root root 3.2K Jul 25 04:00 /etc/lighttpd/server.pem

I have also followed the instructions in the last message of this thread without succes: https://community.ubnt.com/t5/EdgeMAX/couldn-t-read-X509-certificate-after-reset/td-p/1259997

sudo su -
mv /etc/lighttpd/server.pem /etc/lighttpd/server.pem.crap
add system image ER-e100.v1.7.0.4783374.tar
sync
reboot