PHP error after upgrading to 2.7

Hi Shaon,
thanks for the great plugin! Unfortunately, after upgrading to 2.7 I get a few PHP error messages each time I try to load my page. The page itself works, but could you please have a look at those messages and tell me what to do? The url is http://www.anke-art.de

Thanks in advance
Anke

https://wordpress.org/plugins/download-manager/