lyx file from 2015 not converted to pdf

[Wolfgang Engelmann]

Am 24.03.20 um 19:59 schrieb Jean-Marc Lasgouttes:
> Le 24/03/2020 à 19:41, Wolfgang Engelmann a écrit :
>>
>>
>> Am 24.03.20 um 19:29 schrieb Pavel Sanda:
>>> On Tue, Mar 24, 2020 at 07:21:27PM +0100, Wolfgang Engelmann wrote:
>>>> LyX: Software exception Detected
>>>> LyX has caught an exception, it will now attempt to save all unsaved
>>>> documents and exit.
>>>> Exception: std::exception
>>>>
>>>> Did I understand you correctly, that you did not receive this message?
>>>
>>> No, but you might have assertions enabled.
>>
>> what does assertion mean?
>>
>>> Did you self compile or do you use distribution binary?
>>
>> no, I used the Debian actualizations
> 
> What does Help>About>Information on Compilation say? It should tell us 
> what options where active when configuring LyX.
> 
> JMarc
LyX Version 2.3.4.2
(Friday, February 7, 2020)

Library directory: /usr/share/lyx/
User directory: ~/.lyx/
Qt Version (run-time): 5.12.5
Qt Version (compile-time): 5.12.5

not sure whether I understood: This is the version I am using now. I 
can't tell what I used when I wrote the 2015 Lyx version.
Wolfgang