OK, finally a solution to fix the "system completely unresponsive" problem 100%: Unplug the CMOS Battery for a few seconds, reassemble, plug power in and see the system automatically power up. Downgrade BIOS afterwards. No M+Power, no Crtl+Esc, no playing hide & seek with the right TypeC port and connector orientation. Just plug the power in and enjoy.   Thanks to Dell Berlin & Halle for suggesting the fix.   Now, why did I not try the CMOS battery first thing? After all, it's a known "fix" with such issues... The honest answer: The connector wasn't accessible enough. I was loath to disassemble virtually everything just to get to the connector. 25 screws and most addin cards just to somewhat reach the connector. If I was unwilling to bend the mainboard a bit, I would have had to remove the mainboard completely...   [NAME]:  Whoever designed the CMOS battery connection location was either technically inept or a sadist of the purest form. Same applies to whoever approved the design and the prototypes. At least give me a jumper to interupt the connection, if the connector absolutely has to be virtually unreachable... Having disassembled 7 of those 5520s just today, I'll be sure to include maintenance aspects into future purchases.
About half of our Latitude 5520s no longer boot after having been update from BIOS 1.12.2 to 1.14.3. System won't turn on. Only life signs are the three white LED flashes after resetting RTS clock by pressing down the Power Button for 30ish seconds.   Ticket opened, waiting on technitian at the moment.
Latitude 5520 won't turn on after BIOS Update to 1.14.3