Strange behaviour of a project in Delphi 6 Enterprise /Academic version)

I have a project upgraded from Delphi 5 which stops compiling
immediately, complaining on not finding forms.pas
Funny is that all other projects works nicely, while in this project,
it is looking for the system file in the library path defined at the
end of all pathes used for the libraries (defined in Tools|Environment
Options|Library|Library path). All paths defined are seemingly
ignored.

I accidently opened the project in Delphi 5 some days ago, but I think
the problem did exist before that.

Are there any files belonging to the project that could contain the
problem? I have tried to delete the .pof and the .cfg file with no
help at all. Others that can safely be deleted??