I just noticed that a PDF form containing form data with German umlauts was rendered incorrectly by FormsService (while everything is fine when rendered using OutputService). All umlaut characters were disturbed. It seems that FormsService uses the systems default character encoding which is Cp1252 on my Windows box, even though my XML data file is cleanly is UTF-8 encoded.
I could fix this by appending a -Dfile.encoding="UTF-8" to the CQ_JVM_OPTS defined in the quickstart.bat.
Maybe this helps some other German collegues staring sadly on sentences like:
MÃ¼hlheim-KÃ¤rlicher BÃ¼rger kÃ¶nnten grÃ¼ne GrÃ¼tze rÃ¼hren, fÃ¼r KÃ¶lner wÃ¤re Ã¤hnliches mÃ¶glich.