You might have corrupted the bootloader (perhaps by resetting the device after flashing rc.26 while it was upgrading the bootloader). You’ll need to get OpenOCD >= 0.10.0 and try running something like this:
$ openocd -f interface/cmsis-dap.cfg -f target/nrf52.cfg -c "init; program /path/to/bootloader.bin 0xf4000 verify reset exit"
You can also flash the system firmware in the same manner:
$ openocd -f interface/cmsis-dap.cfg -f target/nrf52.cfg -c "init; program /path/to/system-part1.bin 0x30000 verify reset exit"