wp-toolkit generates "Failed to reset cache for the instance #42" errors

OS: CentOS Linux 7.7.1908 (Core)
‬Product: Plesk Onyx Version 17.8.11 Update #68, last updated on Sept 18, 2019 10:22 PM

We've had these reports from some of our clients, they're receiving emails that look like this:
---CUT HERE---
Subject: <plesk Application Updates.

Updates were not installed for the following items:

1. Website "/staging" (domain1.com.au): Failed to reset cache
for the instance #5

2. Website "/staging" (domain2.net.au): Failed to reset cache
for the instance #27

3...

wp-toolkit generates "Failed to reset cache for the instance #42" errors