I already have posted a detailed description in mahara bug tracker:
https://bugs.launchpad.net/mahara/+bug/1527565
Short version: SSO works flawlessly when only one/a few click on moodle-Block "Mahara" (so all configuration settings seem to be right). When there are more (15) - which is rather standard in a school situation - users wait 'forever' never getting logged into mahara often resulting in 'not available' e.g. Apache 503 errors.
This makes using Mahoodle impossible and is catastrophic as some work has begun there an d cannot be continued.
Both systems fully updated (3.0.1, 15.10.1).
Does anyone know how to troubleshoot such a situation? Only error logging gives php_slow warnings (see link).