In the meantime I got the error on my Linux machine, also. If I try to install a signed xpi (original file, not modified) the error occurs a few times. And if I, for example, add the xpi over “about:debugging”, uninstall it and try to install it again directly, it works fine.
Seems the problem is related to the WebExtension switch. A completely new WebExtension add-on works fine, too. A legacy add-on that was ported as WebExtension, produce this error.