The beta 5 App did not include proper error handling to show what the exact error was. This has been fixed in beta 6.

This login issue happens if the engine connection is unencrypted (if using IP, DNS or domain name without SSL) and if FileMaker Pro has not been permitted access to the files on that host yet.

It is solved by either using an encrypted connection OR permitting FMP access to the files. This can be done by opening the list of hosts from FileMaker Pro, (adding the host of not there) and when prompted, tick “Always permit connection..”, and then connect and close the window. Otherwise, the issue will happen every time FMP is restarted.