Hi Carga,
You are running mojoPortal 2.2.1.8 on Linux right? Please post any issues you find in the linux forum.
I just tested this and it works fine on Windows but I get the same error as you on Mono/Linux. The xml/xsl feature of mojoPortal has not changed much in a long tiime and it worked on Mono in the 1.x branch so it may be a Mono bug or regression. I will investigate it and see.
Thanks,
Joe