This time there is a match:
/var/lib/zentyal/conf/webadmin/psgi-subapps.yaml:{"/ebox":{"validateFunc":"EBox::RemoteServices::RemoteAccess::validate","appName":"EBox::RemoteServices::RemoteAccess::psgiApp","userId":"remote_user","validation":1},"/soap":{"validateFunc":"EBox::RemoteServices::WSDispatcher::validate","appName":"EBox::RemoteServices::WSDispatcher::psgiApp","userId":"remote","validation":1}}
Can you open the ]/var/lib/zentyal/conf/webadmin/psgi-subapps.yaml and comment out this line?. To comment it, just add the '#' cahracter at the begin. Then restart webadmin to check if this is the problem