Mod Security got automatically disabled after updating Plesk to Plesk Onyx 17.8.11 #MU 68 on Ubuntu 16.04 or Debian 8 OS.
- #PPPM-11003 "On Ubuntu 16.04 and Debian 8, ModSecurity can now be again installed and switched on with the Atomic Standard rule set."
- Plesk Obsidian 25 September 2019 (fixed as PPP-44268, Linux)
If update is not possible for some reason you may try the following
Enable ModSecurity via Plesk GUI:
Go to Tools & Settings > Web Application Firewall (ModSecurity) > Select On > Choose a Rule set > Click OK to apply the changes