Some "Appereance" settings non more reachable after 4.7 WP-update

Hello guys, first i wish you all a very good 2017! To my current issue: Environment WordPress: 4.7 Catch Evolution Pro: 3.4.1 + Child Theme PHP-Version: 5.6.29 (web-server) MySQL-Version: 5.5.52 (web-server) WP Memory Limit: 256MB I tried to update my WP-Installation from 4.6.1 to 4.7 and i recognized big problems (see please following link to know more about. https://wordpress.org/support/topic/after-4-7-update-site-is-lost-no-chances-to-re-login/#post-8573467). With a really great help from WP-forum, i could execute the update but now, if i try to set on Appearence -> Customize, i get a blank page (if i go back i go beack to the normally WP-backend). The same strange thing happens if i try to go to Appereance -> "Header" and Appereance -> Background. If i set define('WP_DEBUG', false); on "true" in wp-config.php i can read following error warning: "Fatal error: Allowed memory size of 41943040 bytes exhausted (tried to allocate 86017 bytes) in /home/astroliv/public_html/wp-includes/pomo/streams.php on line 201" The file name .php seems to be not important, every time i call the function it appears another .php-name. Otherwise, If i set the site on the theme Twenty sixteen, i can reach all 3 functions again. Now, i am very doubtful. I am not able to determine if the problem is basically sourcing in WP-kernel or in my theme or someting others... Thank a lot in advance for your suggestions. Kind Regard Ernesto PS. the same request ist posted in WP-Forum (see link at the beginning)