Unable to save settings : IntelliJ IDEA

I get this too (IntelliJ 15.0.5), but only when the "Markdown support" plugin is used. Once a .md file is visualised the error starts appearing.

For future reference current version: 2016.1.20160405.143

Not a very solution-oriented post, but maybe this can help someone further pinpoint the source of the error.

This workaround allows you continue using "Markdown Support".

Uninstall the latest version of "Markdown Support" and install version "8.0.0.20160127.143.1" from https://plugins.jetbrains.com/plugin/7793?pr=idea. Everything works fine.