Do I understand this correctly: you have a project file and photos on in a networked folder; you’ve been using this project with Tropy 1.5; you installed 1.6 and now you’re seeing this error when you try to open the project – is that right?
Tropy can open projects from a remote server: but it needs to be able to access the project file via the file system (it can load the photos via HTTP also, but the project file itself currently has to be accessible via the file system).