Jenkins generates invalid JSON on http://.../api/json url due to Git commit messages are included in changeset comment attribute as is (the control chars like \x00-\x1F are not filtered).
All Unicode characters may be placed within the quotation marks, except for the characters that MUST be escaped: quotation mark, reverse solidus, and the control characters (U+0000 through U+001F).
The example you provided shows a "control-A" character that should be escaped but is not escaped. I believe that character should have been represented as \u0001 instead.