[XeTeX] What's this error: "pdf_link_obj(): passed invalid object." ?

[Thomas Fehige]

Hello again,

I'm occasionally getting cryptic error messages like this in TeXworks' 
console window:

-----------------------------------------------------------------
** ERROR ** pdf_link_obj(): passed invalid object.

Output file removed.
] [10fwrite: Invalid argument
xelatex.exe:
-----------------------------------------------------------------

They go away when I rerun the compile several times. Where do I start 
looking what's going wrong?

(TeX Live 2010 on Windows Vista 64)

Thanks + all the best -- Thomas