I have found a solution. I followed the official guide for file permissions (http://codex.ww.wp.xz.cn/Hardening_WordPress#File_Permissions) and set the ownership for all files and folders to <user>:psacln. The Plesk installation sets the ownership of some files to www-data:www-data.
I am aware of this security issue and have set these permissions only temporarily for testing purposes. There is no one else to contact since I am hosting this WordPress blog on my personal web server.
I just figured it out myself. Seems I didn’t read the changelog carefully enough:
All language css classes are prefixed with “crayon-” to prevent conflicts
Would be nice if user-defined themes would update according to changes in the stock themes in the future!
