nrf_fstorage_is_busy() return always true

Hi all together,

SDK 15.3.0

I have a problem writing to flash with the nrf_fstorage module. I try to write
an array of 32 Bytes into flash, triggered by a BLE write by nrfConnect.
After calling nrf_fstorage_write(), the wait_for_flash_ready() function does
never return, which means nrf_fstorage_is_busy() return always true.
I know the forum posts
and tried the solutions discussed there. I expanded my BLE connection interval
to 798,75 ms (as noted in log of nrfConnect), and I have the
in sdk_config.h.
The functions nrf_fstorage_write() and wait_for_flash_ready() functions are called
from main()-loop, the BLE write command only sets a volatile flag, which is checked
in main().
If I read the flash with nrfjprog --memrd, I see that the array itself is written.
But if I try it a second time, the array is not written any more, which would
meet the observation, that nrf_fstorage_is_busy() stays true.

Thanks in advance