Loop Repository with dates in it throws a "Process failed" message

Dear Rapidminer Community, 

I came across a strange problem today, which I hope you can help me with.

I have ~200 datasets which I wanted to join beneath each other with "Append". The datasets include three colums which have dates in the format dd/mm/yyyy hh:mm in them. 

I tried to build a loop (Loop Repository) which goes through my repository and lists the data beneath each other. I used the same loop I have used previously, although the previous data didn't contain any dates. However, when I tried to loop the data I got this error message and I don't know how to handle it. 

The strange thing is that I only get this message when I used the loop? When I simply retrieve the data and use the append operator it works fine and the data is listed below each other. 

Does anybody know how I can fix this problem? Because I would definitely be better to do this with a loop instead of using ~200 retrieve operators ^^ 

Best regards

Felix