If you are having the issue mentioned in the title after an update to EmberZnet 126.96.36.199 please consider updating to the Gecko Bootloader. If it is not an option for you the following is an explanation on why this problem occurs followed by a proposed solution.
Although support of Ember Standalone Bootloader for EFR32 based products has ended in 2017. There could still be some devices in the field that were initially manufactured with the Ember Standalone Bootloader and are now unable to upgrade to the Gecko Bootloader.
Since the 2.5 GSDK release modules are regarded as first class citizens and while the Gecko bootloader has been upgraded to facilitate this update the Ember Bootloader was not because it is no longer being developed.
This update causes the AAT header in .ebl files built during the post-build process in Simplicity Studio to be interpreted as corrupt by the Ember Standalone Bootloader.
The problem is that the value of byte 0x78 used to be 0x10 for EFR32MG1P or 0x11 EFR32MG1B and after the update it is set to 0x01 for MGM111.
If this is the issue the advised solution is to convert the created .s37 file to .ebl using the 'em3xx_convert' tool.
This should be located at "(StudioFolder)\developer\adapter_packs\em3xx\utils"
Open any command line tool at this location and you can convert a .s37 to .ebl with the correct AAT header using the following command.
"em3xx_convert.exe --chip EFR32 (path)/(filename).s37 (path)/(filename).ebl"