I have wp 5.0.3 and i tested wpforo on 4.x.x too
the wpf-includes/functions.php disapears after a while(shorter than two days)
no more plugins
the oblique theme 1 and 2 tested
functions.php renamed too functionx.php and replaced on line158
and i even removed line 161(if is wpf-admin or something like this) but the functions.php will go with no reason and i have error 500 again i will change to buddypress or something else but i liked foro and i like too know what is happening
@moxen do you have any security plugins like Shield for example that are deleting non core WordPress files ?
@moxen do you have access to the Apache error log ? That may provide some insight as to what's going on here. This doesn't sound like a permissions issue. You may have an infected webserver.
This is probably a hosting problem, and to be honest your hosting company (Hetzner) is not among the good ones.
Is bikaran.org your domain or direct3d.ir ?
All that smell like a hacked server.