ERR_EMPTY_RESPONSE – on a surey-script after it worked very well for weeks

[dil_bert 5]

dear community, good day dear friends, 

have a issue - on a webserver i have two installations - 

a. wordpress
b. limesurvey (see https://www.limesurvey.org ) 

i get the ERR_EMPTY_RESPONSE - on a surey-script - cf http://www.f-s-j.de/limesurvey/index.php/admin/authentication/sa/login

it happens especially in chrome - not so often in FireFox 

Sometimes I was able to access the webadmin section without an issue.  i have  installed LimeSurvey under a folder inside my wordpress installation - i have gotten get all kind of issues over the time.

- MyDomain.TLD/limesurvey/ (LimeSurvey)
- MyDomain.TLD/ (Wordpress)

LimeSurvey and Wordpress tell the server to rewrite URLs for the browser. In my setup Wordpress will overrule certain settings of LimeSurvey. well i guess that i  would need to check every change and update manually the htaccess file (note; i am running apache). To minimize work and trouble, i can install wordpress and limesurvey side by side. then i need to redirect from YourDomain.TLD/ -> YourDomain.TLD/wordpress/, but i have clean folders and don't cause delays and loops when it comes to URL accessing. 

well i just could use /w/ for wordpress and /s/ for limesurvey will keep the URL short.

question: are there further options i have to solve the issue with 

look forward to hear from you

Edited November 20 by dil_bert

[dil_bert 5]

i will have a closer look at the system and if have conflicting htaccess rules then i will need to install them next to each other instead of one under the other one. 

well i need to get more information about errors,  so i need to find Apache's error log and PHP's error log. 

tomorrow  i will have a closer look at the error-logs and the Apache-logs 

i will come back here and report the findings