Traditionally PTXprint has been able to print almost anything you throw at it. But as we have recently added interactive PDFs which require a greater level of 'control' over the text, I am guessing it is tripping up on structural USFM issues later in the file. You could work around this problem by just adding a changes.txt rule like this, for example, to throw away everything after (and including) chapter 2 of MRK, which would leave you with the contents of just chapter 1: (replace MRK with the right book and chapter as needed)
at MRK "(?s)\\c 2\s.+" > ""
If for some reason, you still get the same error even with this change, then try placing this additional line before the rule:
sections("initial")
at MRK "(?s)\\c 2\s.+" > ""
If you still get an error after that, it indicates that there is some other error (not necessarily a USFM issue) within chapter 1.