%matplotlib qt hangs notebook

Dear,
sometimes when I run %matplotlib qt in the ipython notebook (v4), it is 
indicated that the cell is executed and that the kernel is not busy. 
However, the next cell cannot be executed: there appears a *, as should 
be, but there is no output, the * remains, but the kernel does not look 
busy.

This mainly happens when I run the second cell, while the %matplotlib qt 
command is not finished yet.

Is this a known problem? How can I diagnose what is going on? I tried to 
start jupyter with --debug flag, but it does not give any errors or 
messages relating the notebook/kernel that is giving problems.

This problem occurs at both a windowsxp machine with the newest python3 
version from anaconda, as on a ubuntu machine, with
* python '3.4.0 (default, Jun 19 2015, 14:20:21) \n[GCC 4.8.2]'
* matplotlib 1.3.1

Both with ipython4 from pypi

Kind regards,
Vasco