Optimized kernel problem
Does it make sense to use -O when working with -a 3?
If yes, read further.
If no, ok, roger that.

I am using Hashcat (v4.2.1-7-g6469357c) on a Debian9 with latest nvidia driver.
When I execute
hashcat -m0 [HASHLIST] -w3 -O -o [OUTPUT] --remove --outfile-format=3 -a3 -1 ?l?d ?1?1?1?1?1?1?1?1?1?1 -i --increment-min=6
my system hangs and does a reboot.

No messages are logged in /var/log/.
GDB is of no use, because no core got dumped.
After reboot, I need to manually delete
in order to continue to work.

Only once, I had the following message in syslog:
hashcat[29356]: segfault at 0 ip 000055efca37e495 sp 00007fff1cd27c80 error 6 in hashcat[55efca311000+be000]

When I use the same command without the -O, everything works fine.
When I use the same command with the -O, but with -a 0, everything works fine.

Any idea?
Looks like a temperature or power problem.
You were right, it was a power issue.
Added an extra PSU and now it works like a glove.