I have this old RB2011L-IN which I just changed the power supply because the previous one was dead.

I reset it and upgraded it to the last version (6.43.13), checked all ports and found that when I connect ether5 the USR LED starts blinking the same way as the traffic LED of ether5.
This happens only with ether5, all other ports with same cable don't influence the USR behaviour (off). It seems that ether5 is working just fine.

Does someone have an explanation for this behaviour? Is there something faulty?
I searched for documentation about this but found nothing.
Thanks!

Is there anything configured in /system leds ?

No, it's totally blank.
I am using the default configuration.
Also, the wiki says that on RB2011 it's not possible to change the LEDs behaviour.