For what it's worth, I had the same problem, and fixed it by downloading synoboot.img again from the source. Edited it to match my USB stick, serial and MAC address, and then it worked.
Downloaden it from here: https://mega.nz/folder/yQpw0YTI#DQqIzUCG2RbBtQ6YieScWg/folder/TM4QVAob