When Task Manage isr Running Dolphin wont show its native transfer files dialog no matter if track progress is marked or unmarked. And all applications that use the task manager plugin like lattedock are affected too.
Kai?
I contacted Kai, who said: You have to disable progress monitoring in the settings of the Task Manager and Notifications applets, then Dolphin will show the transfer window.
(He also adds you might have to restart the shell after changing the applet settings, though, due to a bug.)