As a note to someone that may face such issue, I'll leave it here.
I have isolated the switch (rb260gs) and router (rb750gl) then I was able to pin point the issue.
After isolating the router, switch and a single linux workstation, I started to ping the switch from the linux workstation. While the ping was ongoing successfully, still on the linux workstation, I opened a browser and pointed it to the switch's admin web gui (http://switch-s_ip_address). At the exact moment I hit the enter key, the ongoing ping started to fail (missing packets due to the lack of response) and all the port's led lights were blinking which indicates that the switch was rebooting. As soon as I cancelled the request (the x button on firefox's address bar) at the browser, the ongoing ping started to succeed (response came back).
With that I disconnected the router from the switch and connected a raspberry pi with linux (raspbian) on it. So I had the switch and two clients, being a linux workstation and a raspberry pi only. I could ping from any client to the switch and successfully access the switch's admin web gui while both clients were pinging the switch's ip address. The moment I connected the switch to the router, the ping on both linux workstation and rpi started failing, the switch's admin web gui became unavailable again and the switch rebooted itself each time I tried to access its admin web gui.
Checking the rb750gl config I have found the IGMP proxy package enabled and the igmp proxy set in order to provide iptv on a vlan.
This fact brought me to the following posts at this forum:
RB260GS IGMP snooping? -
viewtopic.php?f=17&t=80054&p=400794&hil ... ts#p400794
filter out multicast -
viewtopic.php?f=17&t=54590
So, my fix was to create/apply an ACL rule at the switch (as shown at the second link above) that redirects the multicast packets, with destination ip "224.0.0./4", from the port 1, coming from the router, to any/some specific port/s (port 5 in my case). As indicated on the second link above, if you want to filter multicast packets out, simply select "redirect", but just leave the redirect port numbers deselected.
Screen Shot 2018-03-12 at 19.03.02.png
Now I have the switch rb260gs properly working, including its admin web gui.
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