Just encountered same problem (error while saving, couldn't reopen project) and mostly resolved it -->
Open your .qgs project file with a text editor (e.g. notepad, notepad++, textedit) and it should show the xml markup.
The 'unexpected end of file' is because the application did not finish saving the end of the document, and so there are xml tags that were not closed. Add closing tags where needed, for example my file needed:
It will be easier if you can open a project file that is working with notepad as well to compare the differences.
hope this helps.