It seems that plyer's uniqueid.py file does not work on ios by default. Changing the line uuid = UIDevice.currentDevice().identifierForVendor.UUIDString() to uuid = UIDevice.currentDevice().identifierForVendor.UUIDString resolves this issue.
uuid = UIDevice.currentDevice().identifierForVendor.UUIDString()
uuid = UIDevice.currentDevice().identifierForVendor.UUIDString