@silau It looks good but ldd doesn't always report exactly what is actually happening when you run your exe. It's a good start though. If you really wanted to trace it further you would have to ldd on each of those libs to see what they link. One of them could be using the 5.6 or whatever other version you may have. That being said I find that to be a bit unlikely though.
When I get a little bit of time later today I will run your code using Arch linux and 5.11 and see what happens. That will help determine if maybe it's a bug in Qt (which I highly doubt for something that big).
If it works for me the next steps for you would be to run it in the debugger and step through the working 5.6 one during socket connections and then the broken 5.11 one and see what is different.
Another option is take it down to the simplest form and just use the chat example or something from Qt and see if it works with 5.11 and then try to see what is different.
I'll let you know what I find out on my linux/Qt 5.11 in a bit.
Edit: Didn't get a chance today, was really busy. I will run a quick test tomorrow some time though.