Like alexsch, I am seeing this in Workstation 15.5.6, but in what might be the worst-case scenario. Everything was fine yesterday. At the end of the day, I deleted the one existing snapshot of the VM, which took a while since there were a couple months' worth of changes to roll in. Bad idea.
Today, the guest screen starts flickering immediately after the "logging in" process completes and the flicker period is just under one second. I can't do anything in the guest while the flickering is happening; no mouse cursor ever appears. Sending Ctrl-Alt-Del to the guest brings up the expected screen, not flickering. If I log out, the next screen also does not flicker, but logging back in restarts it. I can also start Task Manager there and it will run, sitting on top of the flickering screen. Navigating through Task Manager is hard because there is no mouse and the keyboard input is only recognized for a very brief part of each flicker.
I tried running VMware-vdiskmanager to check the vmdk file, but it fails with "VixDiskLib: Failed to initialize PhoneHome library." I did a Repair install of Workstation with no improvement.
I would accept a temporary workaround of disabling VMTools, but I don't see how to do that outside the guest O/S.
The following unfamiliar entries in the log are flagged as errors by the Configuration Manager Trace Log Tool:
2020-07-24T10:23:27.670-04:00| vmx| I005: OvhdMem OvhdUser_BusError : 1 1 - | 0 0 -
2020-07-24T10:23:27.793-04:00| vcpu-0| I005: Win32U_GetFileAttributes: GetFileAttributesExW("E:\Virtual Machines\Win10x64 PC4\Win10x64 PC4.vmpl", ...) failed, error: 2
2020-07-24T10:23:27.796-04:00| vcpu-0| I005: Vix: [mainDispatch.c:4200]: VMAutomation_ReportPowerOpFinished: statevar=0, newAppState=1872, success=1 additionalError=0
2020-07-24T10:23:27.853-04:00| mks| I005: SWBWindow: Window #0 validation failed: no valid host window or host surface.
2020-07-24T10:23:27.927-04:00| mks| I005: SWBWindow: Window #0 validation failed: no valid host window or host surface.