Well now this is just dandy - we`re having this issue after updating to SM17/100 build 6970.
We updated on 04 at 00:30 and ran it a full day, then at 21:00 the webmail interface would not show up anymore or when it did the LOGIN_PAGE_MAINTENANCE_BODY error appeared. Rebooting the services did not solve the problem.
Also tried installing the previous build and the build before that - no luck.
From what I observed, all the protocols work (SMTP, IMAP, POP, SUBMISSION, etc) - just the WebMail is missing.
Running the IIS config tool will make the WebMail interface come up for 1 minute, then it`s not accesible at all again.
This is very serious - what else can I try?
EDIT: apparently we were flooded with a SYN attack on port 443 which caused the WebMail interface to come up for max 1 min after a reboot / service restart, but then it would show the LOGIN_PAGE_MAINTENANCE_BODY error or refuse to connect outright. So it is not the same error - sorry.