My main suspect is the QNetworkAccessManager I use, which is only one used all network access, and is only destroyed when the program quits.
I intend to create a simple test program to see if that could be the case.
I also suspected the great number of widget creation/deletion, always managed by a simple layout, but I created a simple program to test that, and, while there seems to be some leakage over time, it didn't seem to be the source of my problems.
I'm still investigating. I'll let you know if I find anything =)