Workflow save error

Nextcloud version 18.0.3

When saving flows getting below error

POST http://{domain}/ocs/v2.php/apps/workflowengine/api/v1/workflows/global?format=json

Request Payload
{“id”:-1585211490817,“class”:“OCA\WorkflowPDFConverter\Operation”,“entity”:“OCA\WorkflowEngine\Entity\File”,“events”:[],“name”:"",“checks”:[{“class”:“OCA\WorkflowEngine\Check\FileMimeType”,“operator”:“matches”,“value”:"/image\/.*/",“invalid”:false}],“operation”:“keep;preserve”,“valid”:true}

Response
500 (Internal Server Error)