I just edited once the config.xml by changing the idletime value:
<entry>
<attribute>idletime</attribute>
<value>7</value> <!-- instead of 5 -->
</entry>
and it worked as it is supposed to but I don't think it was by editing the file that solved my problem.
After some thinking, I came up with another guess: basically, when I first tried, I wanted to activate the autoidle feature and put 10 minutes and leave the autoaway feature off. Since it's the same value for both, I guess that's why aMSN refused to change it even if I don't activate autoaway. The test code must be something like this:
if (idletime < awaytime)
Now, if I put a value below 10 minutes (which the default awaytime) it works.