Windows Vista operating system
To possibly single out reasons: I am running XP/SP3 and LibO 4.0.4 and have not problems with opening of files (Writer, Calc, Impress, Draw) created in 3.6.6 or even 3.5 versions.
@office_mate - Did you observe this on every file? If so may I ask you to file a bug report under
https://www.libreoffice.org/get-help/bug/
It would be nice if you publish the link to the bug report to enable others to read and comment.
I have several files I use everyday, and when neither would open I uninstalled 4.04 and re-installed 3.66. The files opened normally then. So I won’t be able to duplicate the bug as this is not a development computer.
Would it be a possibility for you to install a portable version of 4.0 and use this for testing?