It looks like there have been changes in CTK which broke the file download from XNAT in MITK. Currently the file's ID is used in MITK which seems not to be present anymore. However using the file NAME does work.
[0f7aa9]: Merge branch 'bug-18155-xnat-download-broken-integration'
2015-04-22 13:31:24 Daniel Knorr [caced1]
Added context menu for download in treebrowser.
2015-04-21 16:40:08 Daniel Knorr [97728f]
Using QmitkIOUtil now for reading files.
InternalDownload added in MenuShow.
2015-04-21 15:13:27 Daniel Knorr [14ea2d]
Added context menue in treebrowser. Download-OnActivateNode.
2015-04-21 11:37:23 Daniel Knorr [a5d1b0]
resolved conflicts in QmiktXnatEditor Download
2014-09-29 10:05:32 Andreas Fetzer [06f189]
Use "Name" property for identifying the file to be downloaded. The "ID" property is empty which causes an error during the download
2014-09-29 10:03:58 Andreas Fetzer [60f3c5]
Added missing "/" to download path which could lead to problems during reading/writing files after download
2014-09-25 13:25:56 Andreas Fetzer [2d2753]
Merge branch 'bug-18051-UseMicroServicesForXnatSession'