What a terribly strange and cryptic error message to come across and totally undescriptive, so of course you've Googled it.
The source of the problem is actually quite easy to fix.
Take a look at your RetrofitAPI class. It'll probably look something like this:
Item saveEvent(@Path("id") int id, @Field("type") String type, @Field("title") String title, @Field("details") String details);
Well you're missing one more thing, the @FormUrlEncoded decorator!
That should fix up your problems.