LaTeX3 Error: Backend request inconsistent with engine: using 'pdfmode'

[Guenter Milde]

On 2020-02-23, Kornel Benko wrote:
> Am Sun, 23 Feb 2020 16:28:19 -0000 (UTC)
> schrieb Guenter Milde <milde at users.sf.net>:
>> On 2020-02-22, Kornel Benko wrote:

>> >> >> ...    

>> >> >> > I tested on an updated TL. Removing the custom class option
>> >> >> > dvips fixes it. However, I'm not sure this is the right thing
>> >> >> > to do. In the inverted tests, we have the following: 
...

>> >> If it is/looks right with "auto" now, then it should be save to
>> >> change from "dvips" to "auto" and remove the clause and comment
>> >> from "invertedTests".  

>> > Looks OK for examples/Presentations/Foils.lyx
>> > but not for  examples/fr/Presentations/Foils.lyx
>> > PDF (xetex) (system fonts) OK
>> > PDF (luatex) (system fonts) OK
>> > PDF (luatex) (tex fonts) gives following error

...

>> > 	! error:  (linebreak): invalid list tail, probably missing glue
>> > 	!  ==> Fatal error occurred, no output PDF file produced!  

>> This looks to me like a different problem. We should invert or ignore the
>> "LuaTeX with tex fonts" export (Foils is a rather old package and LuaTeX
>> with 8-bit fonts is not supported by many packages and also seldom used.)

>> How about PDF (pdflatex)?

> This works.

Good.

> BTW, changing the document language from French to English makes the doc compilable too.
> I tried to create an mwe. The error comes as soon as

> 1.) Doc-class FoilTeX
> 2.) Doc-param empty (no dvips, (with dvips has other errors))
> 3.) Doc-lang French
> 4.) tex-fonts
> 5.) output pdf (luatex)

How about DVI (luatex)?

> 6.) ANY text in Title

This seems to be an incompatibility of FoilTeX, babel-French, and LuaTeX.
Hard to tell which party is to blame.

I recommend to invert or ignore this test (as LaTeX error).

Optionally file bug reports with LaTeX MWE to the respective maintainers.

Thanks,
Günter