Summary

When trying to delete a custom field the message is displayed:

"We couldn't delete these fields
Try refreshing the page, and delete them again."

The error in the browser's network tab shows:

"Validation for custom field deletion failed."

It would be good to have a clear message on what is preventing the field from being deleted.

Workaround:

If you're trying to delete a User/Group Picker Custom field and seeing the above WARN in the logs, please use the following KB article to identify the relevant scheme(s) to be able to delete the custom field(s):

Attempting to delete a User Picker or Group Picker custom field shows "We couldn't delete these fields. Try refreshing the page, and delete them again."