I've tried:

Plastic explosives
Dynamite
Crowbar + 10 STR
Multitools

Any idea how to open it?

P.

Did you jam it yourself by picking it and getting a critical failure? A critical failure will jam the lock, which then cannot be opened until the next day (when the clock reaches 0:00 or very shortly thereafter). Save before lock picking to avoid this.

I tried waiting as you suggested, saving, then trying the lock again, but no luck. Just get the msg "You were not able to pick the lock." No luck with lock picks either.

P.

Might need the electronic lockpicks.

Your Lockpick skill might be too low, if after several tries (re-loads) you always get the "You were not able to pick the lock." message. Which door is it, because some of them also open with a COC badge.

57 skill and electronic lockpicks should be enough to opon all the doors in the master's lair (might require many reloads however).