Compiling error

Hi
I’m trying to compile to mobi, and have successfully compiled the book before. I’ve made only very minor tweaks, but now I get this:
Compile_2017-03-30_13-05-16.jpg
I don’t know what the error is - I assume all the metadata stuff is just a record of what was actually being compiled - or is that what is causing the fail?

I got similar messages with a different book, but when I tried again that one did compile OK.

Help!

Gah! Tried exiting Scrivener and starting it up again. The first compile got an even longer list, but the second one worked.
So, immediate problem solved, but I’d still be grateful if anyone can throw a light on what might have been wrong? It is irritating and frustrating when this kind of thing happens.
Thanks