Azure sphere device recovery fails

Question

We have a few Azure Devices that are not responding, and have no idea why.

There are more than a 100+ devices up and running. They can reach the IoTHub, are responding to azsphere commands. However a few of them are sometimes going down - or not even working from the beginning. And we cannot recover them.

$ azsphere device show-attached
HTTPSConnectionPool(host='192.168.35.2', port=443): Max retries exceeded with url: /device/security_state (Caused by ConnectTimeoutError(<urllib3.connection.HTTPSConnection object at 0x7a80a4102160>, 'Connection to 192.168.35.2 timed out. (connect timeout=30)'))

Then when we try to recover:

$ azsphere device recover
Downloading recovery images..
Download complete.
Starting device recovery. Please note that this may take up to 10 minutes.
Board found. Sending recovery bootloader.
Erasing flash.
Sending 16 images. (5615492 bytes to send)
Could not transfer file to device. Please ensure you are using a valid image package or capability file.

We are using the latest SDK.
What can we do?