I am using a theme which has not problem with theme customizer at versteckter Link
But when Toolset 3.4.14 and Toolset Block Version 1.6.1 activated, theme customizer failed to function with critical error as per attached. Other than that, the website frontend and backend works fine.
That will create a debug.log file in your /wp-content/ directory which you can examine in any text editor, but if you are only just turning on debugging now you'll need to re-create the error.