Suddenly SSO configuration failed to login to Mahara with "You cannot be logged in at this time".

Inspection of the moodle peer setup gave an error like "Your stored key is different from the published key" with no output given.

Looking at debug messages gave hint to a (possible) solution published in 

https://docs.moodle.org/31/en/PHP#HTTP_RAW_POST_DATA_errors

adding

always_populate_raw_post_data -1

to /etc/php/7.0/apache2/php.ini on my Ubuntu 16.04 which worked instantly.

Please note that this happened on a moodle 3.2.2+ with PHP 7.0.15 in contrary to the statement on docs.moodle.org claiming this would be a problem in PHP 5.6 only.