Error: No block-level parent found. Not good
That error is coming from dompdf, bundled with CiviCRM.

Here are a couple of references to that error outside of CiviCRM:

https://github.com/dompdf/dompdf/issues/902
https://github.com/barryvdh/laravel-dompdf/issues/389

The error happens for both 4.6 and 4.7, different php, so the trigger to the issue appears to be somewhat removed.

I fixed one case using the pdf setting (here: /civicrm/admin/setting/misc?reset=1). Using the wkhtmltopdf binary instead of the dompdf bundled with CiviCRM solved the issue.

I upgraded to the latest 4.7.27 version with an updated dompdf, and that did not solve the issue, but instead generated a new problem of running out of memory.

I did a bit more research and fixed the 4.6 version of the problem by removing whitespace in the html that is being fed into dompdf, in the file CRM/Utils/PDF/Utils.php, as per the suggestion by benabbotnz in the second issue linked above.