The only thing I've done to this installation is change the config settings that deal with the "PHP Handler not entered" error from another thread in these forums. I've since reversed that setting back to its original as it didnt' solve my problem. In any case, I'm sure that isn't related to this problem.
It is very unlikely that this is related as the setting is for a complete other PHP configuration and binary.
This problem is most likely not caused by any change made in ISPConfig. Did you update anything on your server or have you changed a locale setting in your linux installation?