Pyocd and Picoprobe unable to flash nRF51822 - raspberry-pi-pico

Question

PyOCD and Picoprobe will not flash an NRF51822, exiting with the error cannot read register r0 because core #0 is not halted.
Setup:
Windows 11
Python 3.9.6
PyOCD 0.33.1
Output from pyocd load firmware.hex --target nrf51822:
0001527 I Loading C:\Users\henri\NRF5x\NRF_LED\.pio\build\nrf51822_y5_mbug\firmware.hex [load_cmd]
0006932 C cannot read register r0 because core #0 is not halted [__main__]
Traceback (most recent call last):
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 259, in _enable_read_access
self.flash.init(self.flash.Operation.VERIFY)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 250, in init
raise FlashFailure('flash init timed out')
pyocd.core.exceptions.FlashFailure: flash init timed out
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\__main__.py", line 161, in run
status = cmd.invoke()
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\subcommands\load_cmd.py", line 117, in invoke
programmer.program(filename,
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\file_programmer.py", line 170, in program
self._loader.commit()
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\loader.py", line 289, in commit
perf = builder.program(chip_erase=chipErase,
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 490, in program
sector_erase_count, page_program_time = self._compute_sector_erase_pages_and_weight(fast_verify)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 666, in _compute_sector_erase_pages_and_weight
self._analyze_pages_with_crc32(fast_verify)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 645, in _analyze_pages_with_crc32
self._enable_read_access()
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\builder.py", line 262, in _enable_read_access
self.flash.init(self.flash.Operation.ERASE)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 243, in init
result = self._call_function_and_wait(self.flash_algo['pc_init'],
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 620, in _call_function_and_wait
return self.wait_for_completion(timeout=timeout)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\flash\flash.py", line 616, in wait_for_completion
return self.target.read_core_register('r0')
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\core\soc_target.py", line 218, in read_core_register
return self.selected_core_or_raise.read_core_register(id)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 971, in read_core_register
regValue = self.read_core_register_raw(reg_info.index)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 988, in read_core_register_raw
vals = self.read_core_registers_raw([reg])
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 1009, in read_core_registers_raw
return self._base_read_core_registers_raw(reg_list)
File "C:\Users\henri\AppData\Local\Programs\Python\Python39\lib\site-packages\pyocd\coresight\cortex_m.py", line 1023, in _base_read_core_registers_raw
raise exceptions.CoreRegisterAccessError(
pyocd.core.exceptions.CoreRegisterAccessError: cannot read register r0 because core #0 is not halted
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