Mattermost 6.0 - S3 file upload timeout issue after upgrade

Today upgraded from version 5.4 to version 6.0, all looks good except if the user try to attach a file getting “We received an unexpected status code from the server. (502)”. We are using S3 for file storage.

Reading / downloading the files works without any issue. Tested the connectivity from System Console and it works.

Any help much appreciated