Comment # 15 on bug 1100765 from Wolfgang Bauer

(In reply to . . from comment #14)
> Haven't used compositing/Desktop effects in the past. It doesn't seem to
> make a difference to Spectacle whether compositing is active or not

It does make a difference.
With compositing active, spectacle asks kwin for the screenshot, while with
compositing off it grabs the screen content directly.

The most obvious result of the latter is that hidden parts of a window are
missing.
And there are also problems/bugs in the latter method:
https://bugs.kde.org/show_bug.cgi?id=390787

Obviously, gimp tries to get the screenshot from kwin too (via DBus) if
possible.

> Regarding the 3rd menu option, "Select a region to grab":
> 
> - If I start gimp from the menu (compositing on or off), the 3rd menu option
> IS NOT there.
> - If I start gimp from the command line using dbus-launch (compositing on or
> off), the 3rd menu option IS there.

For the record, the 3rd menu option ("Select a region to grab") is always there
(and works) in gimp 2.8 on my Leap 15.0 system (I don't use Tumbleweed),
regardless whether compositing is on/off or whether I start it from the
application menu or command line (even without dbus-launch).
I also don't get any error message when compositing is off (although
org.kde.KWin/Screenshot doesn't exist then of course).

So there seems to be a "regression" (or maybe intentional change) in gimp 2.10
in that it requires compositing now (when run in Plasma at least).

But TBH, I don't see how the option missing with compositing on can be a Plasma
problem, especially if it also happens when running it from the command line
(without dbus-launch). Apparently it cannot connect to the DBus user session in
your cases for some reason.

What language are *you* using?
The original report states that the problem only occurs with French
(LANG=fr_BE.UTF-8) and not English, though I don't really see how that would
make a difference. (and I did try running gimp 2.8 with LANG=fr_BE.UTF-8 too)

I suppose modifying the menu entry to use "dbus-launch" could be a workaround
for you though.