This has finally been identified and fixed! It is the 2nd fix in the list below
Plex Media Server 1.14.0.5468 has been released as Beta. It has the fix for the transient lockout seen in evidence here. It also has a fix for uPnP auto port mapping
See Release Notice
- Automatic port mapping could fail with some older routers (#9335)
- Fix for Server Unreachable and Remote Access issues (#9228)
Please update to this version to resolve the problem
