This has happened to me for some time.
Every new plugin I try to add in Sonar, I get this error "Encountered an Improper Argument", in popup windows, several times in sequence..
Then I get out of Sonar, I go back in and I do the scan again. And this second time, no error occurs and the plugin is added correctly.
Below my settings.

Although this problem is workable, it's annoying.
I wonder if anyone would know how to solve this?