Cannot get kernel crash dump saved and reboot

I need to get a crash dump of my linux kernel. I am going to describe the scenario: 1. Hardware: Notebook Dell E5450. 2. Linux Distribution: Linux Mint 18.3 (Ubuntu 16.04). 3. SysRq: enabled by /etc/sysctl.d/10-magic-sysrq.conf. 4. Installed packages: linux-crashdump. 5. Grub2: added "crashkernel=384M-2G:64M,2G-:128M" 6. Kdump check: "kdump-config show" reports a working configuration. 7. File system: folder "/var/crash" exists. When I press "Alt+PrnScr+C" the system freezes without reboot and does not save any dump into /var/crash. Same thing sending "echo c | sudo tee /proc/sysrq-trigger". How could I fix this situation? Thank you so much!