Unable to open Design > Templates tab in Web Presence Builder 11.5.6:
Unexpected server error
The following error is shown in
[EMERG] /sitebuilder/JsonApi/ Base_SyntaxException: PHP Warning : json_encode(): Invalid UTF-8 sequence in argument file: /usr/local/psa/admin/externals/Zend/Json.php
This is internal issue that has been fixed in Web Presence Builder 11.5.7.
Upgrade Web Presence Builder through Plesk Autoinstaller.