I dont have a solution but IO cant access the Orca forum at all and I get the same error message. I checked myphp info and server does support :

xslt
XSLT support enabled
Backend Sablotron
Sablotron Version 1.0.2
Sablotron Information Cflags: -g -O2 Libs: -L/usr/lib -lexpat Prefix: /usr

Dunno , stumped