I've run Calibre in debug mode and don't see anything obvious to me in the log, but maybe someone else will. I gave it 8 URLs to download (these:
https://archiveofourown.com/works/11261703
https://archiveofourown.com/works/13...iew_adult=true
https://archiveofourown.com/works/13155444
https://archiveofourown.com/series/673037
https://archiveofourown.com/users/te...s/teaandtumblr
https://archiveofourown.com/works/12657768
https://archiveofourown.com/works/11585544
https://archiveofourown.com/works/12886884)
all of which the kid and I have tried to download singly, together and in various combinations.
The Calibre debug log says:
calibre Debug log
calibre 3.34 embedded-python: True is64bit: False
Windows-7-6.1.7601-SP1 Windows ('32bit', 'WindowsPE')
32bit process running on 64bit windows
('Windows', '7', '6.1.7601')
Python 2.7.12+
Windows: ('7', '6.1.7601', 'SP1', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 5, 5) && EpubMerge (2, 4, 0) && FanFicFare (3, 2, 0)
calibre 3.34 embedded-python: True is64bit: False
Windows-7-6.1.7601-SP1 Windows ('32bit', 'WindowsPE')
32bit process running on 64bit windows
('Windows', '7', '6.1.7601')
Python 2.7.12+
Windows: ('7', '6.1.7601', 'SP1', u'Multiprocessor Free')
Interface language: None
Successfully initialized third party plugins: DeDRM (6, 5, 5) && EpubMerge (2, 4, 0) && FanFicFare (3, 2, 0)
Turning on automatic hidpi scaling
devicePixelRatio: 1.0
logicalDpi: 96.0 x 96.0
physicalDpi: 72.035450517 x 72.0
Using calibre Qt style: True
[0.00] Starting up...
[0.00] Showing splash screen...
[0.06] splash screen shown
[0.06] Initializing db...
[0.23] db initialized
[0.23] Constructing main UI...
EpubMerge: DEBUG: 2018-12-13 13:35:51,973: calibre_plugins.epubmerge.epubmerge_plugin(152): macmenuhack file_path:C:\Users\Elaine\AppData\Roaming\calibre\ plugins\fanficfare_macmenuhack.txt
FFF: DEBUG: 2018-12-13 13:35:51,982: calibre_plugins.fanficfare_plugin.fff_plugin(207): Plugin FanFicFare macmenuhack file_path:C:\Users\Elaine\AppData\Roaming\calibre\ plugins\fanficfare_macmenuhack.txt
[4.32] main UI initialized...
[4.32] Hiding splash screen
[85.77] splash screen hidden
[85.77] Started up in 85.77 seconds with 1992 books
FFF: DEBUG: 2018-12-13 13:37:47,838: calibre_plugins.fanficfare_plugin.fff_plugin(957): FanFicFare v3.2.0