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with 4.3.4 I get

Failed to open a session for the virtual machine Whonix-Gateway.

VT-x is not available. (VERR_VMX_NO_VMX).

Result Code: E_FAIL (0x80004005)
Component: Console
Interface: IConsole {8ab7c520-2442-4b66-8d74-4ff1e195d2b6}

virtualization is enabled in the bios.

This message appears if I start to run my windows guests also.

Remove Hyper-V or any other software that use VT-x

@OP - please post a VM log file if you intend to continue this discussion. See Minimum information needed for assistance.