I am having a similar issue - just installed Central on a DigitalOcean droplet and everything was working fine but I was having issues with email deliverability using the stock email server (couldn't send users signup emails - mail-tester.com wasn't even recieving anything). Followed the directions for setting up a custom mailserver with mailgun and now when I try to log in on the web it throws this error:
Have already tried changing the ports around, secure and not secure, deleting the secure flag altogether - nothing seems to work. currently my config.json.template file looks like this:
What can I do to resolve this issue? TXT and MX records are already verified with mailgun so that shouldn't be the issue either. the issue has to be something with this file but I cannot figure out what it is for the life of me.
EDIT: changed the port to 25 and then I was able to log in, but when I tried to send a test email it just times out (504 error).
Is this a DNS issue? Still stumped.