They are all permissible. If your project is still corrupted, try deleting the <projectname>.cpd file and reopen the project.
Thank you for the tip.
In the meantime i created a new project and am building the project folder/file structure from scratch by creating the new folder and then importing the html files from the old project.
I started the old project by importing a framemaker project and i think there are a lot of residual "stuff" in the windows explorer directory structure that makes it easy to break when shuffling things around from inside robohelp.
By just importing the html files into a new folder in a new project seems to create a cleaner project --and so far it is not breaking.
After I had been working on my project for several hours (wherein i made a few changes to some topic properties), i closed my project and the problem noted previously occurred again barring me from opening the project.
Thanks to your suggestion, I tried deleting the CPD file and i was able to open the project.
Using your tip saved me from having to refer to the backup where i would have lost several hours of tedious work.
Thanks for the tip.