If don't have the JMS multisites version displayed in Joomla this mean that the installation were not completed. Otherwise joomla will copy the version info (this is the latest files copied).

Perhaps you have a timeout during the install.
Try to install Jms multisites from a directory to avoid the time to upload.

I can tell you that we retested the installation of this JMS 1.2.28 on a fresh Joomla 1.5.18 and it working perfectly.
I can also tell you that the update from 1.5.17 to 1.5.18 does not affect any patches in Jms Multisites 1.2.28.

We have already did the update of some customer website to 1.5.18 with full success.

The problem of the blank page is the symptom of a error for you may sometime find additional info in a "error_log" file.

If you want that we setup, install, ... Jms multisites on your environment, we also provide billable support that you can order at
www.jms2win.com/download?page=shop.produ...35&category_id=1