I 'had' a lot of additional icons (e.g. mimetypes) and icon overrides in my <configuration>/resources/images folder - when I start calibre 6 they get removed. My configuration folder is at the location referenced by CALIBRE_CONFIG_DIRECTORY configuration - which normally has a value of C:\_AppData\Calibre\Config_64bit
The attached image shows what I 'have' there after starting calibre 6 (nothing), alongside what I 'had' there from my backup device. If I close calibre 6, then restore the images folder contents from backup, and then start calibre 6 I see the C:\_AppData\Calibre\Config_64bit\resources\images folder gets emptied just before the book list appears.
Here the debug log
Spoiler:
calibre Debug log
calibre 6.0 embedded-python: True
Windows-10-10.0.19044 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.19044')
Python 3.10.1
Windows: ('10', '10.0.19044', '', 'Multiprocessor Free')
Interface language: None
DEBUG: 0.0 GetFileName::__init__.py - loading translations
Successfully initialized third party plugins: CalibreSpy (1, 0, 86) && Count Pages (1, 11, 2) && Drop Search Results (1, 0, 13) && Favourites Menu (1, 1, 0) && Find Duplicates (1, 9, 5) && GetFileName (0, 1, 1) && Job Spy (1, 0, 191) && Modify ePub (1, 7, 3) && View Manager (1, 8, 0)
calibre 6.0 embedded-python: True
Windows-10-10.0.19044 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.19044')
Python 3.10.1
Windows: ('10', '10.0.19044', '', 'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: CalibreSpy (1, 0, 86) && Count Pages (1, 11, 2) && Drop Search Results (1, 0, 13) && Favourites Menu (1, 1, 0) && Find Duplicates (1, 9, 5) && GetFileName (0, 1, 1) && Job Spy (1, 0, 191) && Modify ePub (1, 7, 3) && View Manager (1, 8, 0)
devicePixelRatio: 1.25
logicalDpi: 96.0 x 96.0
physicalDpi: 98.70815939278937 x 98.85405405405406
Using calibre Qt style: True
[0.00] Starting up...
[0.00] Initializing db...
[0.23] db initialized
[0.23] Constructing main UI...
CalibreSpy: initialization_complete
Job Spy has begun initialization...
author_sort_copy_method >>> default tweaks.py method is: invert
current tweaks['title_series_sorting']: library_order
--->>>apply_title_series_sorting_by_library: libname IS tweaked; new value for: Books is: library_order
title_series_sorting changed FROM: library_order TO: library_order
Calibre, and hence Job Spy, was gracefully shut down last time? True
Last time daemon started: never
Last time daemon failed: never
Total daemon starts inception_to_date: 0
Total daemon failures inception-to-date: 0
current library not specified in Job Spy last-viewed tool active library names... Books
Job Spy has finished initialization...
[1.44] main UI initialized...
[1.44] Started up in 1.44 seconds with 8 books
Starting QuickView
BR