Error "Cannot read contents from file.." when changing themes from admin panel

I have found a workaround for this issue. Please copy the folder

\vendor\magento\module-email\view\frontend\email

to

\vendor\magento\module-email\view\adminhtml\email\.

I had to change the theme in Website scope not Global.

Source: https://github.com/magento/magento2/issues/5666#issuecomment-233236951

Try to clean & flush cache, remove generated files, and regenerate static content:

$ php bin/magento cache:clean
$ php bin/magento cache:flush
$ rm -rf var/cache/* var/generation/* var/page_cache/* var/view_preprocessed/*
$ php bin/magento setup:static-content:deploy

Report back if it does not fix the error.