IgorB:

In these days I have tested a few of Cin releases (from 20181231) with
four old computers: two Notebook and two Desktop, all of them with
UbuntuStudio16.04LTS-dvd-amd64 installed.

That is a lot of testing - so thanks for all of that work!
In the two Notebook all the Cin realeses works fine
In the two Desktop all the Cin releases, before 20190430, works fine.
One Desktop pc with Cin-ub16-without-vaapi-x86_64-static-20190509 works
fine; the other Desktop give me a segv message when quit by Cin.

We do not understand the segv message when quit by Cin and could not reproduce.  Did you ever get a dump /tmp/cinelerraxxxxx.dmp file generated?  If not, gg has no more ideas how to debug this.  If you ever do get dump, send it our way.
 
Cin_20190430 release in the two desktop do not run and a error message
"ERROR while loading shared libraries: libva-x11.so.1" occur.
 
Yes, there are a few libva related libraries that have to be installed and libva-x11 is one of them.  They are listed here:      https://packages.ubuntu.com/source/xenial/libva

Finally, I have tried to install UbuntuStudio16.04LTS-dvd-amd64 with
online update in a desktop PC, but the result is the same. I have also
updated that desktop with "sudo apt-get install gstreamer1.0-vaapi", and
the Cin20190430 worked but sometimes the screen flickers. I guess it
also depends by video card.

Not sure what you mean about "the result is the same".  Do you mean that libva-x11 is missing?  If so, see the entire related list at the last URL to see all of the packages that are needed.

I don't know if it is possible but could I build a Cin release (a last
release) more or less it worked before 20190430?

You can do a "GIT" for any previous version.  How to do this is outlined in section 2.1.3 (create an initial clone) and 2.1.4 (build from a previous GIT version).  If you have questions about that, just ask.