[Rob557]

I gave it a quick try and now see how the log file appears in the Command window and how to copy the result ... when Calibre freezes, nothing further is shown in the log window identifying that it froze, but there do seem to be other warning messages during the boot process that may be relevant:

Microsoft Windows [Version 10.0.18363.1440]
(c) 2019 Microsoft Corporation. All rights reserved.

C:\WINDOWS\system32>calibre-debug -g
calibre 5.12 [64bit]  embedded-python: True is64bit: True
Windows-10-10.0.18362 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.18362')
Python 3.8.5
Windows: ('10', '10.0.18362', '', 'Multiprocessor Free')
Interface language: None
Failed to initialize plugin: 'C:\\Users\\D\\AppData\\Roaming\\calibre\\plugins\\Barnes & Noble.zip'
Traceback (most recent call last):
  File "calibre\customize\ui.py", line 703, in initialize_plugins
  File "calibre\customize\ui.py", line 61, in load_plugin
  File "calibre\customize\zipplugin.py", line 293, in load
  File "importlib\__init__.py", line 127, in import_module
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "calibre\customize\zipplugin.py", line 192, in exec_module
  File "calibre_plugins.barnes_noble.__init__", line 11, in <module>
ImportError: cannot import name 'quote' from 'urllib' (C:\Program Files\Calibre2\app\bin\python-lib.bypy.frozen\urllib\__init__.pyc)
Failed to initialize plugin: 'C:\\Users\\D\\AppData\\Roaming\\calibre\\plugins\\DeDRM.zip'
Traceback (most recent call last):
  File "calibre\customize\ui.py", line 703, in initialize_plugins
  File "calibre\customize\ui.py", line 61, in load_plugin
  File "calibre\customize\zipplugin.py", line 293, in load
  File "importlib\__init__.py", line 127, in import_module
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "calibre\customize\zipplugin.py", line 185, in exec_module
  File "calibre\customize\zipplugin.py", line 181, in get_code
  File "calibre_plugins.dedrm.__init__", line 132
    except Exception, e:
                    ^
SyntaxError: invalid syntax
Failed to initialize plugin: 'C:\\Users\\D\\AppData\\Roaming\\calibre\\plugins\\FictionDB.zip'
Traceback (most recent call last):
  File "calibre\customize\ui.py", line 703, in initialize_plugins
  File "calibre\customize\ui.py", line 61, in load_plugin
  File "calibre\customize\zipplugin.py", line 293, in load
  File "importlib\__init__.py", line 127, in import_module
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "calibre\customize\zipplugin.py", line 192, in exec_module
  File "calibre_plugins.fictiondb.__init__", line 11, in <module>
ImportError: cannot import name 'quote' from 'urllib' (C:\Program Files\Calibre2\app\bin\python-lib.bypy.frozen\urllib\__init__.pyc)
Successfully initialized third party plugins: Count Pages (1, 11, 0) && EpubMerge (2, 11, 0) && Find Duplicates (1, 8, 4) && Generate Cover (2, 0, 1) && Goodreads (1, 5, 1) && Modify ePub (1, 6, 2) && Quality Check (1, 11, 0) && Reading List (1, 7, 1) && Resize Cover (1, 1, 0) && Search The Internet (1, 8, 1)
Turning on automatic hidpi scaling
devicePixelRatio: 1.0
logicalDpi: 96.0 x 96.0
physicalDpi: 101.6 x 101.6
Using calibre Qt style: True
[0.00] Starting up...
[0.01] Showing splash screen...
[0.39] splash screen shown
[0.39] Initializing db...
[3.50] db initialized
[3.51] Constructing main UI...
{'default': False, 'width': 400, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': False, 'width': 400, 'height': 680}
{'default': False, 'width': 420, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': False, 'width': 420, 'height': 680}
{'default': True, 'width': 435, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': True, 'width': 435, 'height': 680}
{'default': False, 'width': 450, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': False, 'width': 450, 'height': 680}
{'default': False, 'width': 460, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': False, 'width': 460, 'height': 680}
{'default': False, 'width': 480, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': False, 'width': 480, 'height': 680}
{'default': False, 'width': 520, 'height': 680}
Resize Covers::rebuild_menus - size_data={'default': False, 'width': 520, 'height': 680}
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
libpng warning: iCCP: known incorrect sRGB profile
EpubMerge: DEBUG: 2021-03-10 20:19:11,970: calibre_plugins.epubmerge.epubmerge_plugin(158): macmenuhack file_path:C:\Users\D\AppData\Roaming\calibre\plugins\fanficfare_macmenuhack.txt
[13.44] main UI initialized...
[13.44] Hiding splash screen
Starting QuickView
[15.55] splash screen hidden
[15.56] Started up ...

I'll check tomorrow whether any other clues emerge. Thanks.