I just compared your files with the same files from the SMF 1.1.11 Install package.
You're using an old version of SplitTopics.template.php. The up-to-date one is marked version 1.1.11 (in a comment at the top of the file) and contains the necessary session ID. There are no other differences, so you can go ahead and replace it with the one from the SMF 1.1.11 Install or Large Upgrade package and the error should go away.
It seems you missed this change when you updated your forum (manually, I guess?)
As a side note, versions of SMF files are updated only when the files themselves are updated, not every time there's a new SMF version. So the SplitTopics.template.php you have is marked version 1.1 in all SMF versions from 1.1 (Final) to 1.1.10. The file was updated for SMF 1.1.11, so the file version was bumped to 1.1.11.
This is something to keep in mind when you check the core SMF files on your forum against current versions. You can do this from Admin > Forum Maintenance > "Check all files against current versions." Out-of-date files will be shown with version numbers marked in red.
If any of the shown categories (Sources, Default Templates, Languages Files) is shown in red, clicking on the category name will expand it to show all files under that category.
The Sources category lists all core SMF files in the Sources directory. Default Templates are located in Themes/default/. You can find Language Files in Themes/default/languages/. Just so you know where to look.
