Withings notification about server

I have a Withings BPM and installed the integration. It works, providing sensors for systolic and diastolic pressure, but every day or so I get this email from Withings:

We cannot reach your server to a notification (timeout of 2 seconds). Below the details

Userid : 40752457
Url : [redacted]

Please fix your url. We will delete this notification in 20 days. We will not remove the notification if the next calls are correct.

How can I fix this? Thanks.