Stuck at Loading 100% [UE4.27.1]

Is anyone aware of possible causes and solutions for a stuck or frozen at Loading 100%?

Sometimes there is a delay after loading before an app actually launches but what is an acceptable time to wait? 1 minute? 10 minutes?

Extremely important that I fix as this is for a project going live in the editor for a client.

Using latest UE 4.27.1