I'm having the same issue, and agree that it may have something to do with the file size. I am working on a book of daily readings, and each file is a month's worth ~ 30 pages. It doesn't matter what month I open in code view, if I switch to book view I get the error. These are months that have been long done and validated in previous versions of Sigil.
It appears that when I save in code view it is adding an extraneous carriage return at the end of the file.
That also seems to be the complaint of this idle bug report:
https://code.google.com/p/sigil/issues/detail?id=2237
I may chop my book down to size so the developers have a sample of the problem, which is what they are asking for.
Sigil 0.7.2 QT 5.0.1 build: 2013.05.02