Might even be a machanical problem (pocket lint stuck in the slot ? SD card not perfectly aligned in the slot holder ?)
But at this step, the format on the card doesn't even play a role. The Xperia doesn't even start to talk to it.
[nemo@Sailfish ~]$ dmesg | tail [ 265.430632] PM: noirq resume of devices complete after 1.110 msecs [ 265.433830] PM: early resume of devices complete after 1.410 msecs [ 265.442184] as3668 6-0042: Resuming AS3668 [ 265.477820] PM: resume of devices complete after 43.972 msecs [ 265.479863] Restarting tasks ... done. [ 265.499059] Resume caused by IRQ 240, 1000000.pinctrl [ 265.499071] Resume caused by IRQ 203, 601d0.qcom,mpm [ 265.499080] Resume caused by IRQ 200, qcom,smd-rpm [ 265.499088] Resume caused by IRQ 711, bcmsdh_sdmmc [ 265.499299] PM: suspend exit 2017-10-15 09:21:47.526513235 UTC [nemo@Sailfish ~]$ dmesg | tail [ 275.255907] PM: noirq resume of devices complete after 1.117 msecs [ 275.259264] PM: early resume of devices complete after 1.501 msecs [ 275.266701] as3668 6-0042: Resuming AS3668 [ 275.304143] PM: resume of devices complete after 44.861 msecs [ 275.305928] Restarting tasks ... done. [ 275.336900] Resume caused by IRQ 240, 1000000.pinctrl [ 275.336912] Resume caused by IRQ 203, 601d0.qcom,mpm [ 275.336920] Resume caused by IRQ 200, qcom,smd-rpm [ 275.336929] Resume caused by IRQ 711, bcmsdh_sdmmc [ 275.337557] PM: suspend exit 2017-10-15 09:22:02.056013703 UTC