Mikrotik LHG HP 5 (RBLHG-5HPnD) install woes

Hello all,

(sorry - moved to the right Category)

This question has already been posted in the AREDNmesh forums but since it is also related to OpenWRT, reposting here

I have two Mikrotik LHG HP 5 units. One installed fine, the other doesn't want to accept squashfs-sysupdate.
As suggested, I tried with multiple PCs and multiple browsers (also in incognito mode etc.)

Pxeboot and all that is working fine and as expected but loading squashfs-sysupdate fails with XHR request timeout.

Tried applying the workaround with editing /etc/config/uhttpd and setting http_keepalive to 0 - no change.

Finally noticed something interesting in Wireshark. The log is full of TCP Retrasmission and TCP Dup ACK entries.
in AREDNmesh, simply opening the cgi-bin/status page to load the firmware throws TCP retransmits. Once I hit upload, the logs are flooded with TCP errors
So far, this seems to point to a network issue - BUT - booting the unit in RouterOS and then uploading a file to the unit doesn't show any TCP errors. 10MB file can be uploaded without any retransmits or errors. This makes me believe that Network and HW are fine.

Setup is PC -> 5 port unmanaged 10/100 MBs switch with only the LHG attached to it. Nothing else. Wireshark is quiet with the exception of the usual arp and windows noise.
Also tried on a Linux PC with direct connection from PC to Router. Same behavior but tcpdump doesn't show the TCP errors like Wireshark

The LGH came with 6.49.9 initially. Meanwhile, I've upgraded the RouterOS to 7.15.3 but still the same issue.

Has anyone observed similar behavior? how can I overcome this?

Thanks