Thanks Toni
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After changing the "Graphics API" to DX11 in Miscellaneous, now the "default_overlay.png" is available in the Filter menu
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Running WinUAE displays a Windows message complaining about a missing KB (the 2670838), then the KS1.3 rom image is displayed but no overlay present. Pressing F12, I see that the "Graphics API" has been reverted to DirectDraw.
So problem here is not WinUAE related.
Last time I updated the windows setup on the Netbook I am using now, it was a pain making the Notebook working again (finally full reinstall from scratch)... so... will stop my test here for the moment as I have no time to "repair" this computer one more time in case of a bad surprise after another KB update... Many different issues after Win7 KB updates on various computers made me suspicious and careful with Win7 updates. Worst one were BSOD .