What version of PHP does the site have? Can you edit your PHP configuration? I found this page: https://github.com/splitbrain/dokuwiki/issues/1869 that suggests that updating the open_basedir setting to be:
open_basedir = “/dev/urandom:”
in your php.ini file will unlock the access to the random file for PHP, but that this issue only happens in versions of PHP older than 7.
I’ve never had this issue, but I figured it’s worth testing.
Let me know if you get it working.