PulseAudio failed to init context

[sunrider]

Host: Ubuntu 16.04
Guest: Ubuntu 16.04

updated VBox from 5.2.16 to 5.2.18;
installed extpack 5.2.18;
rebooted host;
start guest, get error as below;
install guest additions 5.2.18 in guest;
shutdown guest;
restart guest VM, get same error as before (log excerpt):

00:00:00.801653 Audio: Initializing PulseAudio driver
00:00:30.807983 PulseAudio: Failed to initialize context (state 5)
00:00:30.808320 Audio: Initialization of host backend failed with VERR_AUDIO_BACKEND_INIT_FAILED
00:00:30.808478 Audio: Initializing NULL driver
00:00:30.808523 VM: Raising runtime error 'HostAudioNotResponding' (fFlags=0x0)
00:00:30.808538 Console: VM runtime error: fatal=false, errorID=HostAudioNotResponding message="Host audio backend initialization has failed. Selecting the NULL audio backend with the consequence that no sound is audible"

As the error msg says there is no guest audio playing through the host.

Host audio itself is ok, i.e. all audio functions on host are intact.

[sunrider]

VBox Warning Popup-WIndow

[sunrider]

Ubuntu kernel 4.15.0-32 on both, host and guest.

[sunrider]

It appears that this error occurs only on the FIRST VM that is started on the host. Subsequent starts of other VMs (while the erroneous VM is running) do not run into the error. And if, while such subsequently started VMs are running, the first VM (with the error) is re-started, it does not see the error on the restart.

[sunrider]

On Ubuntu 16.04 kernel 4.15.0-33 the error appears as follows:

If the host OS (Ubuntu 16.04 kernel 4.15.0-33) has not generated any audio output after login, ALL starts of VMs (in any number or sequence) produce the error as described in the original posting. The error is independent of the guest operating system. (Seen with Linux and Windows guest systems.)

Once an audio output has been generated on the host system (e.g. playing a local video file that contains audio), subsequent starts of VMs do NOT see the error.