Lightstreamer Server invokes the JDK's getCanonicalPath with the path that represents the root of its Internal Web Server (only used to run the preinstalled demos).
The submitted path is determined by the <pages_dir> configuration element in lightstreamer_conf.xml.
If you use this configuration, please check the path specified.

If you don't use this configuration, the Server adds "../pages" to the path of the directory that contains the lightstreamer_conf.xml.
This should point to the "Lightstreamer/pages" directory in the installation package.
I can't figure out why the operation should fail, but since the operation involves the local file system and depends on the underlying OS, several failure conditions are possible.

Please try installing a Server 7.3.3 from scratch instead of migrating an existing 7.2.0 installation and check if you still see the issue.