The Frontaccounting distribution seems to be missing specific font definitions for TCPDF. In the class.pdp.inc file there are references for fonts dejavu and freesans but they are not included in the fontdir. I installed a language file with UTF-8 locale and this means that Frontaccounting wants to use the dejavu-font.
I donwloaded the TCPDF-distribution and installed the specific fonts from it into the fontdir under frontaccounting. NOTE: I have changed 'dejavu' into 'dejavusans' in class.pdf.inc to make it work as there is no dejavu font in the TCPDF-distribution, only dejavusans and some others.
Please consider correcting this for the current and/or next distribution.