Apperantly my host ( strato.de ) added some new bullshit option called "PHP Boost", description of which is "PHP codebase cache, up to 110% speed benefit by PHP scripts execution". Turning it off solves the problem, this "feauture" was not annaounced in any way, and when adding it they've turned it on by default. I Have no idea what exectly it does ( theres is no significant spped difference felt, suerly no 110 % ) and how it works but appereantly it breaks more complicated scripts than "echo $imAWebDeveloper; ". Anyways, im glad that problem is solved and the reason is found and i hope this topic will be useful for other ProcessWire users by the same host.
Mitko's post in Suddenly Random 500 Interal Server Errors was marked as the answer