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The memory setting of the server was to low.
The provider fixed it.
I activate the WordPress Debug-Modus.
When I activate the Enfold-Theme, this message is shown:
Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 20480 bytes) in /usr/www/users/stbugo/wp2023/wp-content/themes/enfold/config-templatebuilder/avia-template-builder/php/base-classes/class-popup-templates-base.php on line 104