Continuing the discussion from Failure to Remove Indents on First Lines of Paragraphs–Please Help!:
Given the wide latitude of possibilities here, it would be impossible to troubleshoot this without very specific information. Firstly it would help to know what the error text actually is, and secondly what the CSS looks like if you open it up in Calibre’s editor and go to the indicated line in the .css file.
My guess is that it is a bug though, rather than something you may be able to fix—though since not everyone sees the bug, it might be something you can avoid by using a specific different set of options. Who knows at this point though, we’ll have to see where it is coming from.