This happens only in 1 particular project and subsequent new projects, existing projects get me resources for Accountable and Assignee. What am I doing wrong with this project??

Please find below the error messages in chrome console

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Failed to load resource: the server responded with a status of 500 (Internal Server Error)

Failed to get /openproject/api/v3/projects/20/available_assignees: HttpErrorResponse

ERROR TypeError: Converting circular structure to JSON
   at JSON.stringify ()
   at polyfills.afa80d9dad72624fb2db.js:1
   at P (polyfills.afa80d9dad72624fb2db.js:1)
   at P (polyfills.afa80d9dad72624fb2db.js:1)
   at polyfills.afa80d9dad72624fb2db.js:1
   at t.invokeTask (polyfills.afa80d9dad72624fb2db.js:1)
   at Object.onInvokeTask (vendor.5c8617f9ff1ced32a64f.js:1)
   at t.invokeTask (polyfills.afa80d9dad72624fb2db.js:1)
   at e.runTask (polyfills.afa80d9dad72624fb2db.js:1)
   at g (polyfills.afa80d9dad72624fb2db.js:1)