Hi
I have not been able to fully undestand why Flexi makes a difference between when sh404SEF is enabled and when it's not, that sounds like a bug in Flex. However the root cause is that some of the categories have a zero value set for "Taille max. du titre des contenus"
For instance the category "f2f + D&S" has this:
As the item "siam-taekwondo-la-rencontre" belongs to that category, it's content is cut off at zero length and replaced by "...". The default value is normally 200 characters which is the case in the other categories that work.
You will need to review your categories settings and be sure the Maximum length is either set properly or left empty as is the case for working categories, for instance:
So the series of event is:
1 - Without sh404SEF, FlexiContent does
not read the category settings properly
2 - With sh404SEF, FlexiContent reads the category settings properly
3 - On your site, the category settings were incorrect for some categories (set to zero instead of empty or a valid number)
It will be up to the FlexiContent team to find out why they are not properly reading the category settings with Just Joomla SEF, it is too deep inside of FC logic for me to spend that much time.
There's also another problem here which is to understand why that "Taille max. du titre des contenus" is set to zero on some categories, maybe some import or FlexiContent update in the past?
Best regards