Warning from MCUboot: Not enough space to run swap upgrade #nrf52832 #mcuboot #fota

Dear All,
I'm using the nRF Connect SDK and try to do a FOTA upgrade to my nRF52832, with MCUboot installed.
After I downloaded the binary via nRF Connect Android App to the device, i get "<wrn> mcuboot: Not enough free space to run swap upgrade".
And the device continues to start with the old image.
Here is my setup:
- nRF Connect SDK v1.2.0 (Zephyr v2.1.99, mcuboot v1.4.99)
- nRF52-DK (nRF52832)
- Windows 8.1
I did not modify the flash partitions in the device tree (default values from nrf52_pca10040.dts) and also no changes in MCUboot configuration (boot/zephyr/prj.conf).
These are the steps I did:
2. Build it with west (which automatically builds the bootloader and signs my application as far is I understood).
3. Flash the merged.hex (which includes also the bootloader) to device with JLink
4. Do some changes and build an update of my application. This is the output (tail) of "west build -b nrf52_pca10040":
[230/241] Building C object zephyr/kernel/CMakeFiles/kernel.dir/poll.c.obj
[231/241] Linking C static library zephyr\kernel\libkernel.a
[232/241] Linking C executable zephyr\zephyr_prebuilt.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:      229348 B     232960 B     98.45%
            SRAM:       57060 B        64 KB     87.07%
        IDT_LIST:         168 B         2 KB      8.20%
[233/241] Generating linker_pass_final.cmd
[234/241] Generating isr_tables.c
[235/241] Building C object zephyr/CMakeFiles/zephyr_final.dir/misc/empty_file.c.obj
[236/241] Building C object zephyr/CMakeFiles/zephyr_final.dir/isr_tables.c.obj
[237/241] Linking C executable zephyr\zephyr.elf
[238/241] Generating zephyr/mcuboot_primary.hex
[239/241] Generating zephyr/mcuboot_primary_app.hex
[240/241] Generating ../../zephyr/app_update.bin, ../../zephyr/app_signed.hex, ../../zephyr/app_test_update.hex, ../../zephyr/app_moved_test_update.hex
[241/241] Generating zephyr/merged.hex
5. Perform the update with nRF Connect App and app_update.bin. Transfer worked well but after the device resets, I got the following output with the previously described warning:
*** Booting Zephyr OS build v2.1.99-ncs1  ***
[00:00:00.003,326] <inf> mcuboot: Starting bootloader
[00:00:00.009,704] <inf> mcuboot: Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
[00:00:00.020,385] <inf> mcuboot: Boot source: none
[00:00:00.026,184] <inf> mcuboot: Swap type: test
[00:00:00.356,567] <wrn> mcuboot: Not enough free space to run swap upgrade
[00:00:00.684,509] <inf> mcuboot: Bootloader chainload address offset: 0xc000
[00:00:00.692,474] <inf> mcuboot: Jumping to the first image slot
I know, the FLASH usage of 98% is bad. Interestingly enough, when I build in Segger Embedded Studio, FLASH usage of 43% is displayed. Is this because I only see the size of my application (not the merged one)?
When I disable some modules to decrease the memory footprint, the FOTA update works well! But then my application has not it's full functionality.
Has anyone some ideas I can do? Do I need to modify the device tree or the mcuboot configuration? Why does it not work, altough the FLASH is not 100% full?
Any help is appreciated.