How to transform a cancelled lead in a confirmed lead

Hey there, 

I created an event and somehow a few people are listed in the participant list of another event. 
So I took them out and inserted them in the right event, but I cannot change their status from cancelled to confimed. Every time Marketo seems like changing the status after I klicked on it, but nothing happens. In the lead´s activity report there is an error message.

I also tried to change the status in Salesforce but this didn´t help either. 

My colleague created a logic which says that every lead is allowed to take part in the event system only once, so first someone is confirmed and cancelled afterwards and than he cannot go back to confirmed. 
But I changed this too => every lead should now be allowed to change the status easily. 

Where is the error? Can somebody help me? I´m very thankful about any idea
Regards, 
Sonja