@Ezequiel Bértola
'exact same problem' should then render debugging info exactly the same. So turn on debugging + use FireFox console to collect information to share here. Let's see if it is 'exactly the same' or not.
In 3.9.x the code/behavior for selecting an activity or resource has changed. Rather than 2 drop down list from which to select, one popup with both activites and resource tools listed from which to select.
So in other posting, op found an in compatible plugin ... how about your site? All plugins up to date?
You can change the new back to the old by adding a line to your config.php file:
$CFG->modchooserdefault=0;
After making that change might be a good idea to purge caches.
'SoS', Ken