Jailbreak Users: Unfortunately the news is not good. Current JB version is currently unstable.
In an effort to stop mapping and hashing services, Niantic has added a significant amount of security measures.
Included in these measures is a Jailbreak detection (similar to what is used in Fortnite). It is not easily bypassed.
You will be presented with a "The device, OS, or software is not compatible with Pokemon Go" message on the screen. This by no means affects your account.
You can attempt signing out and in again until successful again.