Random sD termination

Hello Community ! I’m trying to fix an issue which getting me crazy for more than a month…
I have a small architecture like the one in the image. Two HAProxy & keepalive, and a farm on Nextcloud app behind.
The architecture seems to be ok, but sometimes (often in fact) Nextcloud app freezes and I find a sD termination in HAProxy logs, which match my HAProxy timeouts (server).
Important : the architecture lives in Proxmox node architecture (all machines are in fact VMs).
Any help will be appreciated.