After I did the last update in Arch, OpenVPN3 client stopped working and gave me the following error:

** ERROR ** Failed calling D-Bus method UserInputProvide: GDBus.Error:net.openvpn.v3.sessions.error: Backend VPN process has died.  Session is no longer valid.

I found that this error is still not fixed, but you can easily do a workaround.

So why does this error happen?
It seems there is a problem in glib2 v.2.76.1 (which is what I got updated recently) and it generates a Null in one of the functions OpenVPN uses for the authentication. So, after authenticating, that error appears and client crashes.

How to fix it?
While we wait for a better solution, one easy fix for this is to downgrade glibc to version 2.74.6. Just be aware that this has to be a temporary downgrade while a fix is available, and that downgrading might break your system.

To downgrade, you can install the downgrade command from the AUR, and then do the following:

sudo downgrade 'glib2=2.74.6'

This will downgrade your glib to that version which works perfectly with current OpenVPN 3. Once connected, I advise to upgrade again to current glib2 to avoid possible errors in other apps which depend on newer version.
You can upgrade using the same downgrade tool with:

sudo downgrade 'glib2=2.76.1'

This will probably pop a window which lets you select that package from the cache. Just reinstall and you're good to go.

I have used this as a temporary fix, and I hope OpenVPN will get updated soon to support current glib.

Also, I read in some blogs that this issue also happens on other Linux distributions, but I don't use them so I can't provide a solution for them. This fix of downgrading should work on any Linux, let me know if you do it on other distros.

Hope this helps someone.
Regards,
Damian