Qt5 brings Json classes that are pretty powerful. As any new feature, it may have some bugs lurking in corner cases.

One of this, is when a Json document is created with somewhat big arrays of elements (e.g 2500 elements, each one looks like:
{
"name": "John",
"surname": "Doe 4"
}

Attached an example to reproduce this bug (plus the generated backtrace). Since it depends on the number of elements (and the number of attributes), I wouldn't be surprised if it is related with a stack issue (is QJson using recursive methods?).