Hello Flav,
To truly understand what happens, you need to run:
# kaltlog
from the shell while making the action and look at the errors.
One simple guess would be that the config file for these players is physically missing from the file system, which you can check with:
\ # mysql -ukaltura -p$DB1_PASS kaltura -h$DB1_HOST
mysql> select conf_file_path from ui_conf where id=23454792;
+----------------------------------------------------------+
| conf_file_path |
+----------------------------------------------------------+
| /content/generatedUiConf/23/454/ui_conf_23454792__12.xml |
+----------------------------------------------------------+
1 row in set (0.00 sec)
mysql> Bye
root@jessex:/opt/kaltura/app/alpha/web/quiz-demo# ll /opt/kaltura/web/content/generatedUiConf/23/454/ui_conf_23454792__12.xml
-rwxrwxr-x 1 www-data kaltura 8446 Dec 22 12:38 /opt/kaltura/web/content/generatedUiConf/23/454/ui_conf_23454792__12.xml*
Replacing 23454792 with your own ID.
That said, you don't really need the default players for anything, you can just as easily create a new one from Studio and customize it to your liking instead, nothing overly special in the templated ones anyhow:)