Reproduction Steps:
  1. Have a User Create a Field
  2. Have a Favro Administrator Disable the User Account
  3. Try to Edit the Permissions of the Field to include other Users
Expected: Other User is added without Issue
Actual: Other User cannot be added (Warning Message: User has insufficient access to the workspace. The required access level is guest or above. Current access level for the user is disabled.)
As the Creator of a Field cannot be Removed from the Editing Permissions the only workaround is to Reactivate the User, Edit the Permissions and then Disable the User again. Ideally you would be able to Remove the Original Creator and Reassign it to an Existing User that still has Member Permissions (or Favro Administrators)