Create a new document and leave it blank.
Open Data Merge Manager (DMM), add a source and click on 'Source > Select'.
With the 'Open File' window open click in the DMM on 'Close'. This obviously closes the DMM but leaves the 'Open File Window' open. This itself seems to be wrong to me.
In the 'Open File Window' click on a data source file and open it. APub vanishes after a second.
I tried this several times with different source files. It seems to be consistent.
d.