XSL-FO is a markup language for Formatted Objects (PDF, RTF etc). Everything that is printed in the document should be output within a valid fo:block.
<fo:block> Content here </fo:block>
Mendix only performs the transformation from your models to XSL-FO (.fo file). This .fo file is then exported to the desired output type. You can't edit the .fo file directly, so you have to reverse engineer what throws the error.
Try setting the DocumentExporter lognode to 'trace' and see what the output of the .fo file is. Then inspect what is wrong, i.e. are you trying to print a variable that's blocked by security, is it an empty object/variable etc. Position 55:148 means the 55th line, 148th character in the .fo file.