Need help to configure "fullresolution"

[Ohrwerk]

I have been trying to configure a different screen resolution for 2 days.
Am I doing something wrong or is it just not working?
I would like to start a box in full screen with a resolution of 1280x960.
My GPU and screen is set to display resolutions other than the maximum centered without stretching. This works fine with the old original DOSBox or other applications but unfortunately I can't do it with DOSBox-X.
I hope someone can help me with the crucial tip.
Thank you

Configuration:
I load two configuration files.
File 1: Default DOSBox-X.conf (only country settings and MOUNT to tools drive)
File 2: Configuration for my game DOSBox-X.autoexec.conf
All settings are made in "file 2" to leave the default configuration untouched.
I load these with the options: -conf "..@DOSBox-X\DOSBox-X.conf" -conf "..\Elite Plus\dosbox-x.autoexec.conf"

Config "File 2"

[sdl]
fullscreen        = true
fullresolution    = 1280x960
output            = opengl
maximize          = false
autolock          = false
autolock_feedback = flash #beep
sensitivity       = 100
mapperfile        = ..\@DOSBox-X\mapper-dosbox-yz.map

[dosbox]
autosave                  = 
saveslot                  = 1
savefile                  = 
saveremark                = true
memsize                   = 16

[render]
aspect         = false
aspect_ratio   = 4:3
scaler         = normal2x
glshader       = none
autofit        = false

[video]
vmemsize                  = -1
vmemsizekb                = 0

[vsync]
vsyncmode = off
vsyncrate = 60

[cpu]
core               = auto
fpu                = false
cputype            = 286
cycles             = 970
turbo              = false

[voodoo]
glide         = false
splash        = true

[gus]
gus           = false

[speaker]
pcspeaker    = true
pcrate       = 65536

[dos]
xms          = true
hma          = true
ems          = true
umb          = true
ver          = auto

[config]
set path    = Z:\;Z:\SYSTEM;Z:\BIN;Z:\DOS;Z:\4DOS;Z:\DEBUG;Z:\TEXTUTIL;Y:\TOOLS;Y:\NC

[autoexec]
@ECHO OFF
mount C "..\Elite Plus\GAME FILES"
MIXER MASTER 50
MIXER SPKR 100
MIXER SB 100
MIXER FM 100
c:
rem MENU