Can't edit contacts - "Failed to upload item"

I recently tried to modify a contact that is on a CardDAV server hosted by However, whenever I edit a field and save, I get an error: Failed to upload item “Test Contact”

Some testing details:

  • It seems that I can create a contact and modify from eM Client with no problems, but if I create or modify one directly in Fastmail, I get this error every time I try to modify it from eM Client.
  • I have can delete the contact with no error
  • I have two different computers with eM Client, and both have this problem
  • I can successfully modify contacts from my Android phone connected to the CardDAV server.