All 3 panics are in
com.apple.nke.l2tp
The first 2 panic with
udp_lock
where udp is the UDP network protocol
and the 3rd panics with
soclose
Also in networking code
You have a 3rd party kernel extension
com.promise.driver.stex 5.1.62
which for panics like this is the most likely suspect
Even if com.promise.driver.stex is not networking related, it is possible it is not properly managing it allocated kernel memory and stepping on memory it released but did not stop using.
A very remote possibility is if you have 3rd party RAM installed in your Mac mini. but the consistency of 3 panics happening in the networking code is really not the kind of thing that can be blamed on failing 3rd party RAM, so I'm really going to stick with the com.promise.driver.stex as being the more likely cause.