Hi Martin,
Generally speaking project files made in previous versions of Terragen should load properly in the latest version of Terragen. Of course some nodes may have new or additional parameters but those additions do not typically result in the issues you're describing.
Are you able to load the projects in an "old" version of Terragen, and if so, what version of Terragen loads them properly? What version of Terragen is not loading the projects properly?
Would it be possible to gather a project file and share it with us via our support email so we can take a look at it and see if we encounter the same issue and determine is there's a solution?