Andreas Brodowski (aka @dw2412) has found a Bug in the fetchmail Plugin that could potentially brick a working installation.
More Information and Solution to this bug is found on Github: https://github.com/olia-dev/kopano-webapp-fetchmail/issues/3
I’ve just released a new Version 1.0.1 that fixes this bug: https://github.com/olia-dev/kopano-webapp-fetchmail/releases/tag/1.0.1
I strongly recommend that you either install Version 1.0.1 or delete the file …/fetchmail/php/include/changeAESKey.php.
Thanks again to @dw2412 for making me aware of this bug.