Magento Update, Amasty Full Page Cache and Redis Session can kill your store

Akitogo Team, 16 Feb 2018

While the last updates of Magento 1.9 have been flawless the latest update to caused an error. Backend worked, correct version of Magento appeared in the footer, but shop frontend showed 404 Page not found!
What happened? Error trace was also strange. Default Shop cannot be loaded or is not configured. Did Magento forgot how to load the default store? Very unlikely. The further trace looked normal. Amasty Cache, RedisSession, some logs, everything as expected, but no default store anymore.