I upgraded from artful to bionic (just run apt update && apt upgrade and rebooted before testing the below) and since then my sennheiser pxc550 was immediately disconnected after a connect.
Jun 05 15:18:49 PC-SUJ kernel: Bluetooth: hci0: last event is not cmd complete (0x0f)
Jun 05 15:18:50 PC-SUJ pulseaudio[2773]: [pulseaudio] backend-native.c: Device doesnt exist for /org/bluez/hci0/dev_00_16_94_22_21_D8
Jun 05 15:18:50 PC-SUJ bluetoothd[1033]: Headset Voice gateway replied with an error: org.bluez.Error.InvalidArguments, Unable to handle new connection
Jun 05 15:18:50 PC-SUJ pulseaudio[2773]: [pulseaudio] bluez5-util.c: Information about device /org/bluez/hci0/dev_00_16_94_22_21_D8 is invalid
Jun 05 15:18:50 PC-SUJ bluetoothd[1033]: Endpoint replied with an error: org.bluez.Error.InvalidArguments
Jun 05 15:18:52 PC-SUJ dbus-daemon[889]: [system] Rejected send message, 1 matched rules; type="method_return", sender=":1.108" (uid=1000 pid=2773 comm="/usr/bin/pulseaudio --start --log-target=syslog " label="unconfined") interface="(unset)" member="(unset)" error name="(unset)" requested_reply="0" destination=":1.23" (uid=0 pid=1033 comm="/usr/lib/bluetooth/bluetoothd " label="unconfined")
Jun 05 15:18:53 PC-SUJ pulseaudio[2773]: [pulseaudio] bluez5-util.c: Information about device /org/bluez/hci0/dev_00_16_94_22_21_D8 is invalid
Jun 05 15:18:53 PC-SUJ bluetoothd[1033]: Endpoint replied with an error: org.bluez.Error.InvalidArguments
Jun 05 15:18:55 PC-SUJ dbus-daemon[889]: [system] Rejected send message, 1 matched rules; type="method_return", sender=":1.108" (uid=1000 pid=2773 comm="/usr/bin/pulseaudio --start --log-target=syslog " label="unconfined") interface="(unset)" member="(unset)" error name="(unset)" requested_reply="0" destination=":1.23" (uid=0 pid=1033 comm="/usr/lib/bluetooth/bluetoothd " label="unconfined")
Jun 05 15:19:07 PC-SUJ kernel: Bluetooth: hci0: last event is not cmd complete (0x0f)
Jun 05 15:19:23 PC-SUJ kernel: Bluetooth: hci0: last event is not cmd complete (0x0f)
Jun 05 15:19:39 PC-SUJ kernel: Bluetooth: hci0: last event is not cmd complete (0x0f)
[...same message a few more time until I had a new version installed ...]
I since then applied the patch from https://bugzilla.redhat.com/show_bug.cgi?id=1534857 -> https://marc.info/?l=linux-bluetooth&m=151801178209679&q=raw to bluez (apt source bluez -> patch -> dpkg-buildpackage ... > sudo dpkg -i ../*.deb) and now my headset works again.
---
ApportVersion: 2.20.9-0ubuntu7.1
Architecture: amd64
CurrentDesktop: GNOME
DistroRelease: Ubuntu 18.04
InstallationDate: Installed on 2017-11-09 (209 days ago)
InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Release amd64 (20171018)
InterestingModules: rfcomm bnep btusb bluetooth
MachineType: LENOVO 20FWCTO1WW
Package: bluez 5.48-0ubuntu3
PackageArchitecture: amd64
ProcKernelCmdLine: BOOT_IMAGE=/@/boot/vmlinuz-4.15.0-22-generic root=UUID=128d75b0-64fc-468a-95fc-0aaf631bf4fe ro rootflags=subvol=@ quiet splash cgroup_enable=memory swapaccount=1 vt.handoff=1
ProcVersionSignature: Ubuntu 4.15.0-22.24-generic 4.15.17
Tags: third-party-packages bionic
Uname: Linux 4.15.0-22-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm bluetooth cdrom dip docker lpadmin plugdev sambashare sudo systemd-journal wireshark
_MarkForUpload: True
dmi.bios.date: 06/07/2016
dmi.bios.vendor: LENOVO
dmi.bios.version: R07ET67W (2.07 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20FWCTO1WW
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40705 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrR07ET67W(2.07):bd06/07/2016:svnLENOVO:pn20FWCTO1WW:pvrThinkPadT460p:rvnLENOVO:rn20FWCTO1WW:rvrSDK0J40705WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad T460p
dmi.product.name: 20FWCTO1WW
dmi.product.version: ThinkPad T460p
dmi.sys.vendor: LENOVO
hciconfig:
hci0: Type: Primary Bus: USB
BD Address: E4:A7:A0:14:18:46 ACL MTU: 1021:4 SCO MTU: 96:6
UP RUNNING PSCAN ISCAN
RX bytes:4394393 acl:135 sco:0 events:618416 errors:0
TX bytes:524957211 acl:614106 sco:0 commands:2823 errors:0
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