Uwe,
Yesterday I reported a similar issue:
OpenVPN AS web interface does not work after upgrade to v. 2.9.1.
In my case both user and admin interface are not accessible as they both use the same base web site and the same SSL certificate.
As my OpenVPN AS runs on a Debian Linux which I can administer, I investigated a bit the log records in
/var/log/openvpnas.log and found an error that indicates a problem with the SSL certificate used on the web interface.
-- rpr.