Wordpress notifications are received: Failed to reset cache for the instance #902: An error has occurred when decoding JSON: Decoding failed: Syntax error

Plesk for Linux kb: technical ext: wptk

Applicable to:

WordPress email notifications are received by Plesk admin. A similar error can be found in /var/log/plesk/panel.log when clicking on Plesk > Wordpress > example.com > Refresh.

CONFIG_TEXT: Website Example (https://example.com): Failed to reset cache for the instance #902: An error has occurred when decoding JSON: Decoding failed: Syntax error
Cloning a WordPress using WordPress Toolkit fails with the error:

CONFIG_TEXT: Failed to reset cache for the instance #902: An error has occurred when decoding JSON: Decoding failed: Syntax error

CONFIG_TEXT: Selected items were updated with errors:
* Unable to update plugin 'elementor', details: The plugin(elementor) has not been updated due to unrecognized reason
Attempt to get theme/plugin list will print PHP code instead of JSON list of themes:

# plesk ext wp-toolkit --wp-cli -instance-id 902 -- theme list --fields=name --format=json --quiet --skip-themes=false | head

# plesk ext wp-toolkit --wp-cli -instance-id 902 -- plugin list --fields=name --format=json --quiet --skip-plugins=false | head

Here is how to confirm the issue is not caused by WPTK from Plesk and by plugin/theme:

LINUX:

Change current dir to website document root:

# cd /var/www/vhosts/example.com/httpdocs/
Download official WP-CLI from Wordpress: https://wp-cli.org/#installing
Run command on behalf of system user and using PHP version from domain use "theme list" or "plugin list" depending on what you want to check:

# sudo -u example /opt/plesk/php/7.4/bin/php ./wp-cli.phar theme list --fields=name --format=json
If any html code output except JSON body like below returned this means you have buggy plugin/theme:

CONFIG_TEXT: [{"name":"flatsome"},{"name":"twentytwenty"}]

WINDOWS:

Download to website document root wp-cli.phar https://wp-cli.org/#installing
In CMD switch to website document root:

C:\> cd C:\inetpub\vhosts\example.com\httpdocs\
Run command to get plugin/theme list:

C:\> "C:\Program Files (x86)\Plesk\Additional\PleskPHP73\php.exe" wp-cli.phar plugin list --fields=name --format=json
If any html code output except JSON body like below returned this means you have buggy plugin/theme:

C:\> [{"name":"flatsome"},{"name":"twentytwenty"}]

Plugin or theme are not compatible with wp-cli.

Contact the plugin or theme developers for assistance.