Confirmed in 1.6.1, VMs will now timeout on launch way more often despite working normally. The problem appears to have been caused by 86Box build 2102 which was compiled with GCC 9.3.0.

Since this is outside of my control, I can only work around this problem for now by ignoring the return value of WaitForInputIdle() if a valid main window handle can be obtained anyway. Implemented in version 1.6.2.