HTTP 403 on images loaded from googleusercontent.com
Within several Google API's (like the gmail API for example), Google uses HTTP403 and/or HTTP429 in order to ratelimit certain requests over certain time periods. I do not know what method you are using, if you are using some sort of API etc, nor do I know how busy or large your webapp is. But rate limiting or fair use compliance could be coming into play.
Gmail API Rate Limit Info Source - https://developers.google.com/gmail/api/v1/reference/quota