I never tried Secure WebDAV, so I can't tell you if the validation error occurs because of the secure connection, I never experienced the same behavior, but I sounds plausible that the problem occurs because of the secure connection.
You can always ask Michael Stapelberg, the author of the plugin, for information, you can contact him via his site http://michael.stapelberg.de/webdav-plugin.html.

I'm using the plugin in combination with my own WebDAV server (apache2), it works very well, but I know nothing of the behavior of the plugin with other servers and services.

I'm quite sure there is no other plugin or application that let you access WebDAV shares.
However, Nautilus (the default GNOME file manger) has native WebDAV connectivity. As Maemo is based on Debian GNU/Linux and draws much of its GUI, frameworks, and libraries from the GNOME project, it must be possible to port/whatever the WebDAV connectivity to Maemo.

It won't be me doing the porting though, I'm a social scientist and historian, as you can imagine; I don't have the needed skills to port...