"Fetch failed" and blank pages on shared hosting

Hey guys!

I’m running a multi language site with the admin plugin on a shared linux hosting from 1und1 (Germany).

I randomly get fetch failed popups when using the admin panel. (tested with PHP 5.6, 7.0, 7.1 or 7.2) Also I’m getting completely blank pages, with no errors or whatsoever. Both seem to happen complete random in front- and backend. Nothing is logged, debugger shows no faults.

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I tried playing around with .htaccess settings, updating vendors, disabling plugins, decrease image sizes, but nothing seems to help

I’m lost here… could need some help.
Site works flawlessy on my local dev environment, using MAMP with PHP 7.1