External Diff Tool - P4Merge starts only with dummy save-options action

On version 3.0.8 P4Merge Diff Tool doesn't start.

1. Open Source Tree.
2. Click on any commit.
3. Click on any text file changed in the commit.
4. Ctrl+D

Expected result:
P4Merge is started and file changes are presented.

Actual result:
Nothing happens.

HOWEVER when I open Tools/Options/Diff and click OK without making any changes, Ctrl+D works fine until next SourceTree restart.
UPDATE: I don't have to open Diff tab. Clicking Tools/Options/OK also "activates" P4Merge.

Below my Diff settings: