Error 500 and Invalid Security Token at the same time

Hi people,

can’t wrap my head around this for some time now.

Error 500 is happening all over the admin interface and Invalid Sec Token from time to time. I have gone through the troubleshoot and tryed everything without success. Grav.log is empty.

Checked with the host but without the result. Got this two errors while I was working through troubleshoot, not sure if it is relevant:

[Thu Oct 25 12:29:38.583411 2018] [:error] [pid 666611:tid 140072334296832] [client] SoftException in Application.cpp:261: File "/home2/pulentco/public_html/index.php" is writeable by group`
[Thu Oct 25 12:53:34.483151 2018] [:error] [pid 666611:tid 140072171042560] (12)Cannot allocate memory: [client] couldn't create child process: /usr/sbin/suphp for /home2/pulentco/public_html/index.php, referer:

Please direct me what to do next?