Sentry via Docker / Source Map Upload Problems

Hi all!
We are running sentry via docker, with a simple setup (as described in the docs).
Everything works really fine, except for Source Map Uploads.
We are using the sentry-cli (both native and via docker), but sometimes the upload of a files fails with an http error code 502.
Sentry is proxied via apache, but the apache error log only says that the upstream server sent an invalid response.
I tried debugging it, but the log doesn’t state any problems. The client says:
[INFO] sentry_cli::api request POST https://<…>/releases/sentry-test/files/
[INFO] sentry_cli::api using token authentication
[INFO] sentry_cli::api sending form data
[INFO] sentry_cli::api > POST /api/0/projects/<…>/releases/sentry-test/files/ HTTP/1.1
[INFO] sentry_cli::api > Host: sentry.<…>
[INFO] sentry_cli::api > Accept: /
[INFO] sentry_cli::api > User-Agent: sentry-cli/1.25.0
[INFO] sentry_cli::api > Authorization: Bearer f52458b8***
[INFO] sentry_cli::api > Content-Length: 10006
[INFO] sentry_cli::api > Content-Type: multipart/form-data; boundary=------------------------468d2166657f9c1a
[INFO] sentry_cli::api < HTTP/1.1 502 Bad Gateway
[INFO] sentry_cli::api < Date: Fri, 01 Dec 2017 15:03:32 GMT
[INFO] sentry_cli::api < Vary: Accept-Encoding
[INFO] sentry_cli::api < Content-Length: 232
[INFO] sentry_cli::api < Content-Type: text/html; charset=iso-8859-1
[INFO] sentry_cli::api response: 502
[INFO] sentry_cli::api body:

502 Bad Gateway

Bad Gateway

The proxy server received an invalid response from an upstream server.

` I blanked out some URLs.

Is there any way to get more debug on this?
A wild guess is that the uwsgi server has a problem handling a lot of requests (we are uploading several source maps at once).
Anyone else had this problem and knows how to solve this?