As a workaround, downgrading ppp back to 2.4.7 makes the VPN client work again.

ArthurBorsboom, yes downgrading works, as ppp has been upgraded to 2.4.8 in core repository too, probably more people are going to face this issue.

Upstream issue: https://gitlab.gnome.org/GNOME/NetworkManager-fortisslvpn/-/issues/37

I rebuilt the package from source with makepkg using the PKGBUILD, and it works fine.

I don't think this is an incompatibility in the plugin source code, but the ppp package have been rebuilt and this package networkmanager-fortisslvpn must be rebuilt too to match the ppp version.

Can this be automated somehow? I guess many packages are needed to be rebuilt once another package gets a bump and arch has somehow a procedure for these kind of stuff, but I'm not sure.

I can confirm, that a simple rebuild without any modification solves the issue. Thanks @Damien

Rebuilding the package did not solve the issue for me:

févr. 05 09:28:54 soad NetworkManager[3858]: /usr/bin/pppd: Plugin /usr/lib/pppd/2.4.5/nm-fortisslvpn-pppd-plugin.so is for pppd version 2.4.7, this is 2.4.8
févr. 05 09:28:54 soad pppd[3858]: Plugin /usr/lib/pppd/2.4.5/nm-fortisslvpn-pppd-plugin.so is for pppd version 2.4.7, this is 2.4.8
févr. 05 09:28:54 soad NetworkManager[3857]: ERROR: read: Input/output error
févr. 05 09:28:54 soad NetworkManager[3857]: INFO: Cancelling threads...
févr. 05 09:28:54 soad NetworkManager[3857]: INFO: Cleanup, joining threads...
févr. 05 09:28:54 soad NetworkManager[3857]: ERROR: pppd: An error was detected in processing the options given, such as two mutually exclusive options being used.
févr. 05 09:28:54 soad NetworkManager[3857]: INFO: Terminated pppd.
févr. 05 09:28:54 soad NetworkManager[3857]: INFO: Closed connection to gateway.
févr. 05 09:28:54 soad NetworkManager[3857]: INFO: Logged out.

I have cloned the repository https://github.com/archlinux/svntogit-packages.git, checked out the branch packages/ppp and run makepkg -sif in the trunk/ directory. Rebooting my system did not help.

Hello. Indeed, downgrading to to 2.4.7 does solve the issue. However, I can't see how simply recompiling without any changes can work, as the upstream sources are hardcoded to bind with ppp 2.4.5, unless a different version is specified. Maybe I am missing something here.

In any case, I'm not sure what is the correct approach for fixing this or when an official fix will be provided, as I can see in the repository that the package is currently orphaned, but in order to make this work for me with ppp 2.4.8, I had to modify the PKGBUILD, right before the '--disable-static' option in line 27, and insert a line with: '--with-pppd-plugin-dir=/usr/lib/pppd/2.4.8 \'.

After that, I simply rebuilt the package with 'makepkg', installed it via 'sudo pacman -U ./networkmanager-fortisslvpn-1.4rc1-3-x86_64.pkg.tar.zst' and I was back in business!

I am attaching the updated PKGBUILD file for reference, having also incremented 'pkgrel' from 2 to 3 to avoid potential clashes with pacman's cached package.

I can also confirm that rebuilding networkmanager-fortisslvpn fixes the package.

@Cyb3rD4d, I guess you have rebuilt ppp package, you should rebuild networkmanager-fortisslvpn in order to make it compatible with new ppp (2.4.8)

asp export networkmanager-fortisslvpn; cd networkmanager-fortisslvpn; makepkg -sri

Make sure you have base-devel installed.

So I assume a rebuild and a bump in pkgrel from a maintainer would fix the issue for everyone.

Fixed by a rebuild by @jelle: https://github.com/archlinux/svntogit-community/commit/5dcee752c4fc4a50a64fbb0614528436ae663710#diff-3e341d2d9c67be01819b25b25d5e53ea3cdf3a38d28846cda85a195eb9b7203a

Yes @aminvakil, I re-built the wrong package!
I confirm @jelle solved the issue.

Thank you all for your help!