This is due to the use of accelerated NICs in HyperV / Azure environments with version 6.0.0
This behavior is due to checks running at startup of the 6.0.0 firmware at upgrade time and all subsequent reboots.
Create the VM network interface as a regular one, and not as an accelerated one.
An enhancement request, SSB-3095, has been raised to add support for accelerated NICS in HyperV/Azure environments.
At the time of this writing, February 12, 2020, there is no ETA on when this will be included in a future release.
Please check the release notes listed with the technical documentation of the Syslog-ng Store Box for added features in new releases.