I'm sporadically getting the following error when someone logs in to upload. Their upload will be terminated and connection closed. This is a 13MB file, so it's not huge. Server is AWS m4.xlarge (4 vcpu, 16GB mem) and doesn't get much traffic.
[32] Thu 26Apr18 01:47:45 - (456922) {0x00c17bc7} SSH_FXP_WRITE: handle: 0x00000012; position: 12763267; 1 bytes
[33] Thu 26Apr18 01:47:45 - (456922) {0x00c17bc7} SSH_FX_OK: Success.
[32] Thu 26Apr18 01:47:45 - (456922) {0x00c17bc8} SSH_FXP_WRITE: handle: 0x00000012; position: 12763268; 1 bytes
[33] Thu 26Apr18 01:47:45 - (456922) {0x00c17bc8} SSH_FX_OK: Success.
[01] Thu 26Apr18 01:47:45 - (456922) SFTP error sending, too many simultaneous client requests. Client has exceeded the server's internal buffers.
[02] Thu 26Apr18 01:57:52 - (456922) Session idle time out
[11] Thu 26Apr18 01:57:52 - (456922) Error receiving file "/sftp/clients/rr/outgoing/reports/export_2018_04_25_20_09_06.zip", aborting (12763269 Bytes)
[02] Thu 26Apr18 01:57:52 - (456922) Closed session
[02] Thu 26Apr18 01:57:52 - (456922) User "rr" logged out