Thought as much. There's a bug in KDE's KDEPlatformTheme plugin that affects Qt programs (
https://bugs.kde.org/show_bug.cgi?id=337491). The short of it is that it injects accelerator shortcuts (&) into the menu text of applications. That menu text is what we (Sigil) use to know which plugin to launch from the menu--so this messes with that functionality (and the functionality of a lot of other Qt applications affected by it).
I added a
workaround for it in Sigil 0.9.8 that seems to do the trick, but unfortunately, the version of Sigil included in Xenial-based repositories is Sigil 0.9.5.
There are a few things you can do.
1) Assign plugins to the plugin menu button shortcuts and launch them that way (you only have 3 plugin shortcut buttons available on 0.9.5, though).
2) Build/install a newer version of Sigil yourself from source.
3) Install a newer version of Sigil from a third-party repository (there are several reputable launchpad repos that provide newer versions of Sigil).
There's also some mention of something that can be added to a "kdeglobals" file (in the above bug report comments), but I cannot vouch for its accuracy or effectiveness.