My hunch .. could be either file permissions problem or the *.sla files have become corrupted.
I would try ..
(a) inspecting properties of *.sla file (user permissions?)
(b) opening the *.sla file in an XML editor and validating it.
I have used XMLCopyEditor to inspect *.sla files.
Last resort .. reinstall Scribus if you have just started today.
Basically retrace steps.
I already tried simply copying the file, to no avail. I can make some changes to the file, like adding a new layer (which gets broken then too). Also I ran xmllint which didn't report any xml problems with the file.
I'll try reinstalling but I would be surprised if that would make a difference, since new files seem to work, and in any case I may hope that no changes are made to the application itself when I'm only using it.
I'm still not ruling out that I simply don't understand something. In other words what does it _mean_ if all tools are grayed out? What should one normally do in such a situation? Is it a credible situtation that I can via the Menu create (e.g.) a line -> Insert -> Line, but not via the tool-bar, nor can I then edit any properties of it?