What could cause this (config) cache issue after upgrading from 2.1.14 to 2.1.15?

after upgrading a shop from 2.1.14 to 2.1.15 I’m getting the following exception when visiting any page via the browser:

1 exception(s): Exception #0 (Exception): Notice: Undefined index:  in /src/vendor/magento/module-developer/Model/View/Page/Config/RendererFactory.php on line 64  Exception #0 (Exception): Notice: Undefined index:  in /src/vendor/magento/module-developer/Model/View/Page/Config/RendererFactory.php on line 64 #0 /src/vendor/magento/module-developer/Model/View/Page/Config/RendererFactory.php(64): Magento\Framework\App\ErrorHandler->handler(8, 'Undefined index...', '/src/vendor/mag...', 64, Array) #1 /src/vendor/magento/framework/View/Result/Page.php(149): Magento\Developer\Model\View\Page\Config\RendererFactory->create(Array) #2 /src/vendor/magento/framework/View/Result/Page.php(139): Magento\Framework\View\Result\Page->initPageConfigReader() 

I suspect there is something wrong with writing the cache. The homepage for example renders fine the first time, and on a refresh this exception is shown. When I then clear the cache and refresh the page shows again. Also, everything works fine when disabling the Magento cache completely.

It happens in both developer and production modes, and also in 2.1.16.

What could possibly cause this?