Hey, recently I've updated a Tiki farm from 1.9.11 to 2.4 without problems. I'm not sure but I think that on my tests I've experienced the same problem and the solution was to clean the template cache. You can do this by removing the files under the templates_c directory except the index.php.
Other thing I had to do was run the script to convert the preferences variable to the new format against all my template files.
Let me know if none of the two ideas above solve your problem.