lyx not a readable Lyx document

[Wolfgang Engelmann]

 >>>>>>> I followed the advice of JMarc >

If you still have the directory where you built LyX, then
   gitpull >(should be git pull)
   make
should be enough (you can add "./autogen.sh" as a second line to play 
safe). > this I omitted

But you have a newer LyX 2.4 somewhere, since you produced a file with it!

JMarc
________________________________


The LyX format itself changes regularly during the 2.4.0dev timeline. 
lyx2lyx is updated simultaneously.

When 2.4.0 is released, the format will be fixed and lyx2lyx (barring 
bugs) will stay the same during the lifetime of the branch.
The only exception is that the last 2.4 version will get an updated 
lyx2lyx that can read the 2.5 format.

Likewise, LyX 2.3.7 will be able to read and produce the 2.4.0 format.

JMarc

___________________________________

and from https://www.lyx.org/HowToUseGIT >

4.  Anonymous Git clone

 From within the directory where you want to place your local copy of 
the sources, do:

  git clone git://git.lyx.org/lyx

This will make a directory lyx and download lots of files into that 
directory. (A fresh copy will need about 400MB. However, you will need a 
lot more space for the compiled version -- up to 2 GB).

You can switch to maintenance branch via git checkout 2.3.x inside the 
lyx directory.
5.  After the first download

If the code is updated, just cd to the lyx directory and say:

  git pull

This will magically download or patch only those files which have been 
changed or added, saving you (and the server) lots of time.

If you just want to check what's changed in your local copy as opposed 
to the repository, do:

  git status
__________________________
git status
Auf Branch master
Ihr Branch ist auf demselben Stand wie 'origin/master'.

Änderungen, die nicht zum Commit vorgemerkt sind:
   (benutzen Sie "git add <Datei>...", um die Änderungen zum Commit 
vorzumerken)
   (benutzen Sie "git restore <Datei>...", um die Änderungen im 
Arbeitsverzeichnis zu verwerfen)
         geändert:       po/ar.gmo ....

keine Änderungen zum Commit vorgemerkt (benutzen Sie "git add" und/oder 
"git commit -a")

_______________________________

 >>>>>>>>> however, after it and after
./configure --with-version-suffix=lyx24n
(I unfortunately forgot --bindir=/usr/local/bin)
make
make install

and running a lyx file I get again:

Warning: 609: Format not supported.
Warning: Quitting.
Error: Dokumentformat-Fehler
----------------------------------------
/tmp/lyx_tmpdir.VqwdxGawWrac/Buffer_convertLyXFormatZYzSIl.lyx ist kein 
lesbares LyX-Dokument.
qt.qpa.xcb: QXcbConnection: XCB error: 5 (BadAtom), sequence: 818, 
resource id: 0, major code: 20 (GetProperty), minor code: 0
Warning: 609: Format not supported.
Warning: Quitting.
Error: Dokumentformat-Fehler

 >>>>>>>>>>>>
I am confused.

Does it mean, I have to get all the git stuff again, because for some 
reason the git pull does not update the git stuff, although it says so 
in the https://www.lyx.org/HowToUseGIT ?

Wolfgang