Plesk recently updated the PHP encoder that they use to protect Plesk extensions and you will get this error if your panel is not fully updated. To fix the error you need to update Plesk to the latest microupdate:
To update Plesk though the Plesk Interface
1. In Plesk, go to Tools & Settings and click Updates and Upgrades.
2. On the Updates and Upgrades page, click Update Components.
To update Plesk from the command line (run as root)
plesk installer update
More information on how to update Plesk
What Plesk update is currently installed on the system?
You can view what version your panel is on the "System Overview" section of your home screen after logging in. Alternatively, execute the following command in a command-line interface:
At the time this article was written here are the latest updates for each branch:
Plesk Onyx 17.0.17 - Update 76
Plesk Onyx 17.5.3 - Update 83
Plesk Onyx 17.8.11 - Update 68