Scheduler is down, all events are lost, in front-end xml err

i am using the scheduler since months and one day ago there is following happened:

in front-end all events are not visible any more
only an error popup with xml is shown, starts like:

<?xml version='1.0' encoding='utf-8' ?><![CDATA[2013...

newly created events are also not shown after reload.

System info:
Mysql: 5.1.66
PHP: 5.3.3
Web server - Apache/2.2.16
Joomla! 2.5.9 Stable

By activating Debug mode in Schedule-Admin -> empty screen in front-end is shown.

Please give any hint what could be the reason or solution

Found the reason - an event was created by the user with the german specific letter - umlaut (ß) - therefore the whole component was empty with thexml pop up.

The issue only occores when USER has such letter in the name.
No issue if the umlaut used within the event text.

The database collation is utf8_general_ci,
the db table with events is also utf8_general_ci
the db table with user names is also utf8_general_ci

How can we solve it? for long-term this is not accaptable to not allow the user to write in the profile the proper user name with local letters.

Any idea?