After hrs of debugging I found the following PHP warning in server logs:
PHP Warning: file_put_contents(/XXXXXX/qa-plugin/q2a-open-login/providers.php):: in /XXXXXX/qa-plugin/q2a-open-login/qa-open-page-logins.php on line 426, referer: XXXXXX
This error coming from following lines of code:
//at the end save a list of all active providers
file_put_contents( $this->directory . 'providers.php',
'<' . '?' . 'php return "' . implode(',', $activeProviders) . '" ?' . '>'
There was no write permissin to the q2a-open-login plugin directory, so PHP code could not save the providers.php file. Once given the write permission everything worked fine. After saving the providers.php I restated the default directory permissions.