lyx not a readable Lyx document

[Wolfgang Engelmann]

I posted this already under
tmp/lyx not a readable file
(01March22)

The error message:

/tmp/lyx_tmpdir.MQUsZMBvAPQl/Buffer_convertLyXFormatwrdYvM.lyx is not a 
readable LyX document.

since quite a number of my older (March 22) lyx files are affected and I 
can't open them, I would very much appreciate if somebody could have a 
look at it.

I found 2 lyx files close to each other, one of which gives the error 
message above, the other one is alright. I could send both privately 
(because of the size and the content) to somebody for checking them.

I use lyx 2.4.dev under Debian.

Wolfgang