You could change the login method to local. So you would be able to login to the webadmin by using a local user. Then you have login to the webadmin and you can go on with debuging.
Login to console. Edit /etc/wfe/conf/proxyauth.ini
Goto the section [webadmin_methods]. Add the word "local". This enables local users to login to webadmin. Then add the word "admin" to the section [webadmin_users]. I assume the user admin exists on your system. Otherwise use a user which exists.
Now you should be able to login to webadmin.
If that still does not work, the Astaro User Authentication deamon (AUA) might not run. This would require further investigations. Just post a message, if you still need help.