error shown on front end and backend

Hello, On the front end the scheduler shows the following error:

Warning: error_log(WP_PLUGIN_DIR/event-calendar-scheduler/scheduler_log.txt) [function.error-log]: failed to open stream: No such file or directory in /home/zorgboer/public_html/components/com_scheduler/codebase/connector/tools.php on line 223

I have found some solutions on this forum but non of them solved the error. On the back end a XML message with data is shown when I open the scheduler configuration.

I noticed that one of the custom fields has dissapeared. Maybe this will cause problems. I cannot add new events, events are not visable in the backend. In the database the records are shown.

Can you provide a solution. Is it possible to reset and start over? There are not many entries so that should not be a problem. … and how can I prevent this in the future?