Due to an excessive amount of temporary files created by various PHP sessions in a very short time (+6GB in two minutes), the underlying fs-bucket for PHP sessions became full. You may have noticed various errors from 'write failed: No space left on device' to some more "random" errors that were caused by this.
Applications using redis as a session backend were not impacted by the session issue. It may have been impacted if your application generates temporary files, which are on the same fs-bucket.
Our clean-up policy of temporary files was not aggressive enough, we'll reduce it to once a day and will continue to monitor if we need to upgrade the current disk space.
This incident started at 11:33 UTC+2 and was fully resolved at 11:41 UTC+2