Im using Matomo since 'piwik' era.
I just updated from version 3.5.0 to 3.6.0 and end up with blank page and log full of same error.
I did what I do always when those things happens:
reuse same db(and delete every autoload.php that installation tells me to)
[Fri Aug 31 07:39:42.203081 2018] [:error] [pid 29745] [client x.x.x.x:64220] FastCGI: server "/var/www/x/y/z/cgi-bin/php5-fcgi-*-80-piwik" stderr: PHP message: PHP Fatal error: Cannot redeclare class Piwik\\Plugins\\Goals\\Columns\\Metrics\\GoalSpecificProcessedMetric in /var/www/x/y/z/web/plugins/Goals/Columns/Metrics/GoalSpecificProcessedMetric.php on line 23, referer: http://x.y/index.php?action=finished&module=Installation
It was same line just after updating.
./console diagnostics:run yell:
ERROR always_populate_raw_post_data = -1
To prevent some critical issue, you must set the following in your php.ini file: always_populate_raw_post_data = -1
After making this change, restart your web server.
Which wasn't the case while reinstalling and is strange because I have it setup that way - so diagnostic check overall server setting probably ?! dunno
After restarting both apache and php5-fpm Im able to login into dashboard but there are still some fatal errors.
[Fri Aug 31 07:58:08.996093 2018] [:error] [pid 30774] [client x.y.z:64428] FastCGI: server "/var/www/x/y/z/cgi-bin/php5-fcgi-*-80-piwik" stderr: PHP message: PHP Fatal error: Cannot redeclare class Piwik\\Plugins\\CoreVisualizations\\Visualizations\\HtmlTable in /var/www/x/y/z/web/plugins/CoreVisualizations/Visualizations/HtmlTable.php on line 23, referer: http://x.y/index.php?module=CoreHome&action=index&idSite=1&period=day&date=yesterday