WordPress Core loading times

Hello,

after two months of working fine, suddenly the page loads a very long time (60 or 120 seconds or timeout). Profiler says it’s the “WordPress Core”. It occurs when i navigate to the “Pages” in WordPress and after some times its fine again.

I tried deactivating all Plugins. Tested with PHP 7.2 and 7.3 with and without php boost (Hoster: Strato). The only thing, that worked for now is changing the Theme to a standard wordpress theme. When chaning back the bug reappears.

Website: https://www.owfd.de

Is this known? Is there a solution? I attached a screenshot of the profiler with all Plugins activated.