LXC container reboot fails - LXC becomes unusable

Solution is a kernel later than 4.15. If you want to patch in a custom kernel, you could use 4.18, but this is unsupported.

It doesn't appear that Proxmox 5 will be released with a new kernel branch considering it doesn't seem to affect everyone and the reasons one system is affected but not another is still not known. However, Proxmox 6.0 is now in beta and available for testing, which contains kernel 5.x, which should resolve this issue.
 
Tested it this day with 6.0. It still works with the LXC's. But wait. I still do not believe it.