Ctrl+D is not working on multiple files

I upgraded from Sourcetree 2.5.x (or something like that, I'm not sure which version I had actually installed before) to version 3.0.8.

Now I'm no more able to compare more than one file at a time with Ctrl+D

I'm using Beyond compare Version 4.2.2 (build 22384) and I configured External Diff / Merge like this with Tool-Options, tab Diff: 

External Diff / Merge:

  Diff command : C:\Program Files\Beyond Compare 4\BCompare.exe

  Arguments: \"$LOCAL\" \"$REMOTE\"

 Steps to reproduce:

• Select one file in the "Unstaged files" pane
• Ctrl+D
• Beyond Compare shows the comparision as expected
• Back in sourctree select another file in the "Unstaged files" pane
• Ctrl+D
• Now Beyond Compare shows another TAB as expected, but apparently the left file could not be opened (the one in the temp directory with a name like C:\Users\Michael\AppData\Local\Temp\9lspNa_MyFile.cpp) .

Selecting more than one one file in Sourcetree and then pressing Ctrl+D doesn't work either, the symptoms are the same as explained above.

This used to work in all previous version s I've used.