Workflow stage was not specified

Describe the issue or problem
When I click on Add Reviewer and give all details, i get “workflow stage was not specified”
Please tell us what happens and what you expected to happen.

Steps I took leading up to the issue
I checked the roles for the user who is to assign reviewer, journal manager role is there. i tried to find some clue in browser console but in vain

What application are you using?

Additional information
Please add any screenshots, logs or other information we can use to investigate this problem.

This error appears when i press Add Reviwer… The server has been shifted to ubuntu and cloudflare https also applied… Any clue please

The same error occurs when I decline the submission. The following is the complete error message shown in browser console:

{"status":false,"content":"A workflow stage was not specified.","elementId":"0","events":null}

Any help is appreciated @asmecher