Your credentials have expired.
I had to do a factory reset on my Galaxy Note 4 (un-rooted running Android 6.0.1).
I've installed the Plex client (188.8.131.5253) and now, every 4 or 5 minutes the app logs me out whith a message "Your credentials have expired. Please sign in again".
I'm not all that technical savvy, but things I have tried include:
Clear the Plex App cache.
Un-install and re-install the app.
Changed my Plex password.
Performed another factory reset on the Note 4, and re-installed Plex.
My other clients (PC, & Smart TV) work fine and if it relevant, my Plex server (Version 184.108.40.20629) is running on a Windows 10 PC.
I've run out of ideas now, so if anyone could give me any pointers I'd be really grateful.
Thanks in anticipation.