It is a known issue with the appstack created by older version of the agent.

Apparently when logging in if not all appstacks are attached it seems as if Windows claims the disks and provides it with a drive letter.

eventually (if you are using 2.12.1) after 180 seconds Windows releaes the driveletter and Appvolumes agent processes the appstack.

There is a quie simple though pretty cumbersome solution to it.

You need to make sure that the appstacks (and yes, you need to update them all) have the NoDefaultDriveLetter setting enabled.

Following link is a how to.

Network drive letters are not mapped to VMware App Volumes AppStacks (2139351) | VMware KB