qrexec-daemon's socket in /var/run/qubes removed

[marmarek]

Qubes OS version
R4.1

Affected component(s) or functionality
qrexec

Brief summary
After some operation (unfortunately I don't know exactly what), I've got qrexec-daemon sockets in /var/run/qubes in dom0 removed, although qrexec-daemon processes itself were running.

To Reproduce
Steps to reproduce the behavior:
1. ??
2. Try to use qvm-run

Expected behavior
Working qrexec / qvm-run

Actual behavior
qrexec-client says connect: No such file or directory, while target domain is still running and its qrexec-daemon process is running too.

Additional context
About that time I've also got this message in journalctl in dom0, for all affected VMs:

qrexec[...]: qubes.WindowIconUpdater+: sys-net -> dom0: error while executing: qrexec-client failed: ['/usr/lib/qubes/qrexec-client', '-d', '@adminvm', '-c', 'SOCKET12,sys-net,5', '-E', 'QUBESRPC qubes.WindowIconUpdater+ sys-net keyword adminvm']

I suspect the socket got removed by atexit(unlink_qrexec_socket) handler in a child process (that called exit() instead of _exit()), but I haven't found which one.

@pwmarcz any idea?