That's quite common actually. I was passing this border a year ago with a van (sprinter), which is definitely not a truck. Besides that I had no goods in it, it was completely empty. After a couple of hours waiting, they told me the same crap about a missing document. I guess I was lucky I can understand and speak a bit their language. At the end they took me about 50 euros.

Anyway, good luck with the rally and have a nice time in Bulgaria.