Product and version: FileMaker Server 16.02.212 (team license, single machine deployment)
OS and version: Windows Server 2008 R2 Enterprise
Browser and version (for WebDirect only): Chrome, Safari, Microsoft Edge
Hardware: CPU: Xeon E620, 12 GB RAM
How to replicate:
Configure a regular instance of FileMaker Server 16 in a single machine deployment on Windows Server 2008 R2 (or 2012, I don't think it makes a difference). Configure a setup with an router that maps a non standard SSL port on its external interface to port 443 on the FileMaker Server. Connect with FileMaker webdirect on the configured non standard SSL port on the external interface of the router.
Workaround (if any):
I removed the 'hidden' class from login container (div with id (login_dialog_wrapper') in the the login.html file in the FileMaker Server installation, in the subfolder with the code that Vaadin uses for the launchcenter. But this clearly is a hack.