So duplicati-bd2d9bf5f4fa54ee4a3d280718c7c8aed.dblock.zip.aes looks like a 2018 file that passed the first scan for NUL rows, but now has a few, and also a bad hash? The header at least looks right though.
If the above is true, maybe the file grew an error somehow. You can check time stamps further, e.g. both ls -l and ls -lc for hints that a program changed it. If not, does QNAP have the smartctl command to check how healthy your drives look, or other metrics? A NAS often has redundancy, but does it log when such redundancy actually has to be used? Linux systems may also log I/O problems to the system logs.
The affected command run as mentioned earlier should be able to tell you the losses of deleted dblocks, and Disaster Recovery as mentioned earlier gives steps for recovery from removal. I suggest you move the dblocks to a different folder just in case something goes wrong and we wind up wanting them again, although the three with the bad header are probably lost. If you like, try manual decrypt and unzip of the one with the good header but bad hash (and small run of NULs) to see whether there’s anything left of it. You can run SharpAESCrypt under mono from the Duplicati install folder. Help text for that is found here. AES Crypt is another option, but you might have to move the bad file to some other system to decrypt it.