504 errors

I'm experiencing 504 errors with API Gateway that to me seem rather excessive (10 errors per 4k requests, aka 0.25% error rate) and I can't explain why I'm getting these 504 errors. I'm doing a straight API Gateway to SNS integration so I don't have any logic of myself in between that would impact the processing of requests.

Anybody got a clue what's happening here and even better how to improve? I can't image that a 0.25% error rate is normal for API Gateway?

Kind regards
Meint