Update KB4343909 for Windows 10 1803 has broken ‘Windows Defender Application Guard’ (WDAG) after installing the August 2018 KB4343909 update.
The Windows Defender Application Guard reports the error code 0xC0370106 as shown below.
We confirm that it is a ‘known issue’ if you read the release notes of KBb4343909 : “Launching Microsoft Edge using the New Application Guard Window may fail; normal Microsoft Edge instances are not affected.”
The workaround is uninstall the KB4343909 update and install updates KB4340917 and KB4343909 in that specific order. Microsoft will fix this in the September release.