Many thanks Kevin, Doitsy and DiapDealer. I really don't know what is the cause of the issue with the official package from Arch; I tried with QTWEBENGINE_DISABLE_SANDBOX=1 and QTWEBENGINE_CHROMIUM_FLAGS="--no-sandbox" without success. So I built Sigil with the very clear instructions by DiapDealer and now I have a fully functional Sigil 1.9.10

Thank you very much again!