The org/bitbucket/cowwoc/diffmatchpatch/DiffMatchPatch class is provided by the diff-match-patch-1.2.jar library in the Lightstreamer\lib\core folder.
Please check if you can see it.
If you can't, perhaps you have updated the Server binaries to version 7.3 in an installation package for version 7.2?

To resume the full list of libraries needed for the Server 7.3 startup, a procedural description seems better:
  1. Download Server version 7.3 from the download page.
  2. Go to the Lightstreamer/lib folder.
  3. Consider all jar files in this folder and all its subfolders, except for the jars included in the log\bridge subfolder.