You've probably got filesystem damage on your EFI System Partition (ESP), which is mounted at /boot/efi in Ubuntu. This type of damage is often caused by the Windows Fast Startup feature, which can do serious damage to filesystems on a dual-boot configuration. It's imperative that you disable this feature when dual-booting with Windows. See this page for instructions on disabling this feature.

After you disable Fast Startup, you may also need to repair filesystem damage on the ESP. You can do this in Linux with the dosfsck command, as in sudo dosfsck /dev/sda1; however, you'll need to know what partition to pass as an argument, as that varies from one computer to another. Type sudo parted /dev/sda print to see your partition table. The ESP can be identified as a FAT partition with its "boot flag" set. Alternatively, you can use CHKDSK or its GUI equivalent in Windows, but I'm less sure of the exact procedure there, so I can't provide details. In a worst-case scenario, you might need to copy all the files off of the ESP, create a fresh FAT32 filesystem on it, and copy all the files back. If you do this, you'll probably have to adjust your /etc/fstab file in Linux to get the partition to mount correctly, since its serial number ("UUID," although it's not technically a UUID) will probably have changed.