Nothing in the error logs.
I think that something is wrong with mod_security. I tried this plugin on three websites: same configuration, same plugins. On the first website the scan doesn't work. On two other websites from another provider this plugin works perfect. So I think some rules from mod_security don't let wordfence scan to work.
Do you have some old reports regarding some issues of this plugin with some rules from mod_security?
Anyway, I'll ask my host providers to check this issue also.
Regards,
Cosmin