VPN l2tp issue with VPN site to site : works on Debian, on 16.04, not on 18.04

I did open this question a few weeks ago already : 18.04 VPN l2tp issue - connects but no access to resources

I'm still investigating but so far here are my findings :

• used to work with 16.04
• works on Debian stretch
• works on Debian testing
• works on Arch
• fails on Mint 18.04
• fails on Fedora 28

It is exactly this issue also : https://bbs.archlinux.org/viewtopic.php?pid=1802711#p1802711

Now, does anyone has the slightest idea how to see differences between Debian on which Ubuntu are based, and Ubuntu ? This is a very tricky network issue, maybe a net guru will step by ...

I've already updated xl2tpd to circumvent the 'kernel 4.15' issues. I've updated anything I could. I've followed 'ip routing' magic trick : no better result.

What the hell can be broken in this distrib ...

regards.