I wrote the Ubuntu image to a USB flash drive using Startup Disk Creator and had the same error during booting it.
Launching Partition Manager I saw that the partition is FAT32. I recalled that I didn't format the partition during image writing -- it already was FAT32 (perhaps from previous Windows installation).
So I deleted the partition using Parition Manager and rewrote the image (Parition Manager showed that the created partition is FAT32 too). It booted fine. I guess something was wrong with the format of the partition