Due to the fact that Symfony configuration has the gc_probability: 1 by default, instead of using the php.ini one of '0'.
This means that there is a 1% chance that the GC process will start on each request.
Since Platform UI make hundreds of requests the 1% probability will occur very fast.
A fix for this problem is to set the gc_probability: 0 in symfony by adding to config.yml the following:
The real problem is related with the fact that by default www-data user, using php-fpm doesn't has permissions to remove the session garbage (/var/lib/php5/session)