Latest picture of the surge protection device installed on the DB.
Best theory at the moment is there is a second PC that is not behind a decent UPS and it's passing whatever is incoming via the land cable connecting it to the router. I'm going to install a wireless link and remove the lan cable next.
The only other "exposed" connection would be the copper line for the (A)DSL connection. Assume if a PC can pass this through a lan cable then so can a POTS line via can a modem?
Has this PC (or network card) or ADSL modem ever burned down by surge? Usually, when such a surge passes through the equipment, something else than mikrotik may burn out. (unless it passed by shields/ground and pc/dsl modem doesn't have ground/ or they was disconnected from mains).
For example in last incident lightening entered server room over asterisk connected to POTS, from card to "ground" of PC, and because it was not properly grounded, passed over whole rack to device that was has connection to ground, burning everything on the way. After i traced, i found biggest damage on Asterisk FXO port. You can see traces of explosion on card under module 3, and exploded MOV(not sure, lazy to check if it is MOV) on removed module, it was not visible, until i removed it.
Was there a thunderstorm when Mikrotik failed? (Most likely yes, just noticed surge arrester charred)
And yes, POTS quite common source of surges, unless they are completely underground all the way and no underground high voltage lines nearby. If it is not underground, you definitely need to put surge protector on POTS, they should not be expensive.
Otherwise surge will come over POTS, pass your DSL modem and then, or over DSL modem supply to ground, or more likely over network to closest device that is connected to ground.
P.S. Also avoid shielded cables and plugs.They work only when properly grounded, and when not - they do exactly opposite.