LyX ⇒ *noob question* LyX won't compile on new system

[flogae]

Hello everybody,

I just ported an old presentation that I last delivered in 2020 to my new computer and I am having trouble letting LyX compile it for me. I could use the old version, but the date on the first page says March 10, 2020, which is a bit embarrassing.

The new computer is running Ubuntu. LyX is opening the .lyx file with no trouble, but upon pressing ctrl+R to compile, I get an error window with the classical

LaTex error: missing \begin{document}

I am not enough of a wizard to tell what might be the problem.
Any ideas? What tests can I run? Something seems to be wrong with LyX's configuration...

[flogae]

It is quite confusing....
I have looked at some of my other older talks, and they all compile without a problem.
So there isn't a general error in LyX's configuration, but rather something in the specific file that doesn't compile.

I thought that since the error I am getting is a

missing \begin{document},

the problem must be in the header section.

But the header sections of the two files are identical, except for a

\usepackage{graphicx}.

I don't see how that could cause the problem, but I'm willing to comment it out, except I don't know how.

Anyway I've attached the files, maybe someone can try to compile these and see what happens?