Thanks for the detailed report! To me this seems like a backend-problem, but it’s quite strange that you get a 502-error, as this often means that the server is not reachable through your reverse-proxy or some other server-setup error. Do you get this consistently and only when you are deleting a data element? Are you able to refresh the page and everything else is working/loading?
Regarding the error-message from the frontend “undefined was not deleted”: this is a small bug in the error-handling code (it’s using “name” instead of “displayname”). The culprit is the 502-error.