Ubuntu 18.04 - "Activation of network connection failed Ubuntu"

I found the solution looking at the syslog

sudo tail -f /var/log/syslog

Apparently the .crt file with the certificate was inside a folder with spaces in the name, and that was causing the error. The moment I removed the spaces from the path to the file, it worked on the first try.

Weird bug, but it solved it.

In my case, I had many Wi-Fi configured that were not available at time hence showing error, just disabled the Wi-Fi and open Wi-Fi at the use of time. Problem solved.