Comment # 5 on bug 953014 from Graham Davis

I've done some further testing and the problems seem more complex than I
initially thought.

Just in case anyone's interested, here are the later tests on another machine
with two versions of 42.1:
Version of Dolphin on 42.1 beta is 15.04.3-3.5 but built against KDE platform
4.14.0.
Version of Dolphin on 42.1 build 265 is 15.08.2-2.1, built against Qt 5.5.0.

Switching from Beta to 265 loses all tabs and preferences.

After setting up new Dolphin windows, including tabs on one, I logged off. I
did NOT get the warning box! 

On logging back on, all the windows re-appeared but without the tabs. I
repeated this test and the same thing happened. 

This sort of behaviour with Dolphin has happened before in KDE4 and was due to
the formatting within the config files being changed so that one version was
incompatible with the other. I don't know whether something similar has
happened here. Unfortunately, I haven't yet found the location of the new
config files.

I'll continue with more tests on this in order to fathom out what the deuce is
going on. In the event that I find out something more conclusive, I'll do as
you say and raise it with KDE.