I faced exactly the same issue. Tried to update firmware to M2T103. The process stucked at verifing stage.
OS Win 10. I was using user (not admin) account but provided admin password when prompted.
This is how I solved the issue.
I noticed that despite monitor was turned off usb hub was still working. I connected my old display and connected U2518D only with power and usb cables. Then I logged on as admin and started the update utility. It showed: "No monitor detected" (or something like that) but Update button was active. I started the update process. This time it finished successfully. I repluged U2518D after update and now it is working fine.
So in order to restore your monitor you need another display for the time of update. Do not forget to login as admin.