Object Schema Managers are not able to add users in the roles

Issue Summary

When a users that is Object Schema Manager (but not an Insight Administrator) tries to add a user to any Object Schema role, a pop-up is raised to the browser with the message:

The Jira server was contacted but has returned an error response. We are 
unsure of the result of this operation. 

REST API

http://localhost:48132/j8132/rest/insight/1.0/user/picker?
showAvatar=true&query=t&exclude=JIRAUSER10200&exclude=JIRAUSER10000&_=1615486692378 

Status code: 403

{"errorMessages":["Sorry, you do not have permission to perform this action.. PermissionInsightException: User JIRAUSER10200 
didn't have correct permission (admin)."],"errors":{}}

Steps to Reproduce

1. Insight Plugin 8.7.6; 8.7.10
2. Log in Jira a user that is Object Schema Manager (but not Insight Administrator);
3. Go to Insight > Select the object schema > Configure > Roles;
4. Try to add any user to any one of the three roles;

Expected Results

Since the user is an Object Schema Manager, it was expected that he/she would be able to manage the roles of the Object Schema and add the user to any of the available roles.

Actual Results

Pop-up with error message appears:

The Jira server was contacted but has returned an error response. We are unsure of the result of this operation. 

Workaround

Use a user that is an Insight Administrator to define the users in each role of the Object Schema.