Okay, I have soldered the second Sensebender too and tried to burn the bootloader via Arduino IDE onto both devices. The verbose log was the following:
/home/x/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino9/bin/avrdude -C/home/x/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino9/etc/avrdude.conf -v -patmega328p -cstk500v2 -Pusb -Uflash:w:/home/x/.arduino15/packages/MySensors/hardware/avr/1.0.1/bootloaders/DualOptiboot/optiboot_atmega328_pro_8MHz.hex:i -Ulock:w:0x0F:m 
avrdude: Version 6.3, compiled on Jan 17 2017 at 11:00:16
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2014 Joerg Wunsch
         System wide configuration file is "/home/x/.arduino15/packages/arduino/tools/avrdude/6.3.0-arduino9/etc/avrdude.conf"
         User configuration file is "/home/x/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping
         Using Port                    : usb
         Using Programmer              : stk500v2
avrdude: usbdev_open(): Found AVRISP mkII, serno: redacted
         AVR Part                      : ATmega328P
         Chip Erase delay              : 9000 us
         PAGEL                         : PD7
         BS2                           : PC2
         RESET disposition             : dedicated
         RETRY pulse                   : SCK
         serial program mode           : yes
         parallel program mode         : yes
         Timeout                       : 200
         StabDelay                     : 100
         CmdexeDelay                   : 25
         SyncLoops                     : 32
         ByteDelay                     : 0
         PollIndex                     : 3
         PollValue                     : 0x53
         Memory Detail                 :
                                  Block Poll               Page                       Polled
           Memory Type Mode Delay Size  Indx Paged  Size   Size #Pages MinW  MaxW   ReadBack
           ----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
           eeprom        65    20     4    0 no       1024    4      0  3600  3600 0xff 0xff
           flash         65     6   128    0 yes     32768  128    256  4500  4500 0xff 0xff
           lfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           hfuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           efuse          0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           lock           0     0     0    0 no          1    0      0  4500  4500 0x00 0x00
           calibration    0     0     0    0 no          1    0      0     0     0 0x00 0x00
           signature      0     0     0    0 no          3    0      0     0     0 0x00 0x00
         Programmer Type : STK500V2
         Description     : Atmel STK500 Version 2.x firmware
         Programmer Model: AVRISP mkII
         Hardware Version: 1
         Firmware Version Master : 1.23
         Vtarget         : 3.0 V
         SCK period      : 8.00 us
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.00s
avrdude: Device signature = 0x1e950f (probably m328p)
avrdude: NOTE: "flash" memory has been specified, an erase cycle will be performed
         To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: reading input file "/home/x/.arduino15/packages/MySensors/hardware/avr/1.0.1/bootloaders/DualOptiboot/optiboot_atmega328_pro_8MHz.hex"
avrdude: writing flash (32768 bytes):
Writing | ################################################## | 100% 0.00s
avrdude: 32768 bytes of flash written
avrdude: verifying flash memory against /home/x/.arduino15/packages/MySensors/hardware/avr/1.0.1/bootloaders/DualOptiboot/optiboot_atmega328_pro_8MHz.hex:
avrdude: load data flash data from input file /home/x/.arduino15/packages/MySensors/hardware/avr/1.0.1/bootloaders/DualOptiboot/optiboot_atmega328_pro_8MHz.hex:
avrdude: input file /home/x/.arduino15/packages/MySensors/hardware/avr/1.0.1/bootloaders/DualOptiboot/optiboot_atmega328_pro_8MHz.hex contains 32768 bytes
avrdude: reading on-chip flash data:
Reading | ################################################## | 100% 0.00s
avrdude: verifying ...
avrdude: 32768 bytes of flash verified
avrdude: reading input file "0x0F"
avrdude: writing lock (1 bytes):
Writing | ################################################## | 100% 0.01s
avrdude: 1 bytes of lock written
avrdude: verifying lock memory against 0x0F:
avrdude: load data lock data from input file 0x0F:
avrdude: input file 0x0F contains 1 bytes
avrdude: reading on-chip lock data:
Reading | ################################################## | 100% 0.00s
avrdude: verifying ...
avrdude: WARNING: invalid value for unused bits in fuse "lock", should be set to 1 according to datasheet
This behaviour is deprecated and will result in an error in future version
You probably want to use 0xcf instead of 0x0f (double check with your datasheet first).
avrdude: 1 bytes of lock verified
avrdude done.  Thank you.
Interesting in this log is the warning about the invalid value for some fuse bits...
Flashing a sketch after re-burning the bootloader sadly did not work, the same not in sync error occured again. Next, I tried to burn the bootloader using the board setting of the Arduino Pro Mini (3.3v, 8mhz). Strange enough, this does not work anymore now, yielding a pretty generic error:
avrdude: stk500v2_command(): command failed
avrdude: stk500v2_program_enable(): bad AVRISPmkII connection status: Unknown status 0x00
avrdude: initialization failed, rc=-1
         Double check connections and try again, or use -F to override
         this check.
I do not know, I can only assume that maybe burning another bootloader does not work because of the invalid value warning while burning the default bootloader. I also tried to burn the original Optiboot without success.