As the person who handles the WWI fate information for this website, I thought I'd comment. A couple of issues here:

Was U 28 sunk as a result off damage caused by the explosion of the OLIVE BRANCH? Yes. That is the official explaination, it's consistant with U 28's patrol area and sinkings.

Is a loss in this manner (damage from exploding steamer) plausible? Yes, for the exact reasons Rainer gives. If U 28 had tried to finish OLIVE BRANCH off with her guns, she would examined the vessel for sometime while submerged; that was SOP in 1917. She would also likely have been closer to the steamer than the lifeboats at that point.

What's about the truck? Now that's where I draw the line. There are a lot of weird WWI U-boat stories (the sea monster, the haunted U-boat etc.) just like there are weird WWII U-boat stories. And just because a story has been often retold doesn't make it true (for example, there's nothing in U 28's KTB about a sea monster, and there was no ghost seen when UB 65 sank...).

Is the truck thing likely to have happened? Probably not. Is it attributed to a reliable source? Also, probably not. The reference is not to Spindler (the source and challenged only with primary source material) but "Under the Black Ensign" by R.S. Gwatkin-Williams. And where did he get his information? Unknown. Is it reliable? Also unknown, but through experience I don't necessary trust British secondary accounts. There's a certain tendence to exaggerate facts or make something into a good story -- and that begins with the Royal Navy, in its monthly intelligence assessments during the war.

Bottom line: believe the truck aspect of this story if and only if it's in the official files on the sinking of the OLIVE BRANCH held at TNA (the former PRO). If anyone's going, I can get the file reference.

Best wishes,
Michael