[DiapDealer]

Yes. The cmake options used when Sigil was built will prove useful.

Also, copying from the clipboard history of the Sigil ini seems like a long way around the barn. Surely the errors present in the PluginRunner dialog and/or the terminal Sigil is launched from as well?

Can I ask why you changed Sigil's default install prefix to /usr? The /usr prefix is typically reserved for packaged software from official repositories. The default prefix when using cmake to compile software is typically /usr/local unless specifically changed.

EDIT: Changing the prefix is probably the issue. If you'd used the default prefix of /usr/local everything should have worked fine. But since you specifically changed the prefix to /usr, the multi-arch lib subdirectory comes into play on Ubuntu (x86_64-linux-gnu). If you do decide to go with the /usr prefix (not something I really recommend), you will probably need to add -DCMAKE_INSTALL_LIBDIR=lib to your cmake configure before compiling Sigil. But it's probably better just to leave /usr to Ubuntu packaged software instead.