Hi!, I had the same problem, and I check to my firmware version, It was RX-51_2009SE_21.2011.38-1_PR_COMBINED_MR0_ARM, but the last version on tablets-dev.nokia.com was RX-51_2009SE_20.2010.36-2_PR_COMBINED_MR0_ARM so, I flashed my phone again with the last official version and then I tried to install Catorise Plus, It works fine!!!
I hope to be helpful
P.D. I don't speak english quite well, so, sorrry for my english...