PDFs can't be created if the description of a project contains more than ~6'000 chars. It stucks at the message "creating file 1 from 1 ... please, wait!"
The problem was in Dompdf tool. The latest version seems working and is uploaded on the test server. All pdf creating files are changed according to it. The New Dompdf can be found in the SVN, as well as changes in ini/params.php and install folder.