@SGaist Hi, and thanks for welcome :)
Sadly I am in a similar situation as the other guy in the thread. We are working on a big project and providing a minimal sample is almost impossible at this moment. Though I understand that without it, it's hard to say what's wrong.
Yes, removing the old data and starting fresh solves the issue, but since many users run our software, it's not an ideal solution. However, increasing the version number, that is passed into this API effectively causes the same fresh start on their machines as well, so I guess this should be enough.
Either way though, it kinda proves the point that from Qt5 to Qt6, something got changed and loading the data from previous version leads to some memory corruption, so I still think this should be solved somehow officially.