Thank you so much. I tried lyx 3.6.0 first as I need it and can always use Twinkle (which I don't much like) rather than the repository version of linphone (3.5.2-which I don't much like, but less).
Lyx 3.6.0 loaded fine and opened files fine, with a big exception: It says that the document class "article" requires "external files that are not available." Now the article class is the main lyx class and comes with the repository's version of tetex. However, after much experimentation, about 6 months ago I decided that texlive, huge though it is, is better than the much smaller tetex, so I removed tetex and installed (selectively) texlive.
The way texlive works is that when lyx says it can't find a document class, I run:
which opens up the texlive internet window and, from there, I download the missing document class.
I tried to do this just now (tried to load the default texlive repository so I can download "article,") but I get the following error:
Loading of remote database failed.
The Tex Live versions supported by the repository
do not include the version of the local installation.
I will stop here. I don't know much about texlive. Perhaps it hasn't yet adapted to the new lyx. But I write because it might be that somehow you could rather easily change the date of your version of 3.6.0 from 2012 to 2013. If so, texlive might find the right default repository. I will be looking into this further as, if there isn't an easy fix, I will have to find lyx version 18.104.22.168 and reload that.
Thank you again. I will look at linphone tomorrow. Late here now.