From that, you are using the batch metadata download. Don't. Do your debugging from the metadata editor. That will show the error and the log will show the full details of the attempt to download the metadata and any errors. That is what @kyalpha used and what I was referring to.
Using your code for a random book in my library shows:
Code:
calibre, version 5.14.0
ERROR: No matches found: <p>Failed to find any books that match your search. Try making the search <b>less specific</b>. For example, use only the author's last name and a single distinctive word from the title.<p>To see the full log, click "Show details".
Running identify query with parameters:
{'title': 'Alien Hunters', 'authors': ['Daniel Arenson'], 'identifiers': {'isbn': '9781505658200', 'amazon': '1505658209', 'ff': 'a/daniel-arenson/alien-hunters', 'fictiondb': 'daniel-arenson~alien-hunters~1197738~b', 'goodreads': '24533763', 'kobo': 'alien-hunters'}, 'timeout': 30}
Using plugins: Douban Books v2 (4, 2, 1)
The log from individual plugins is below
****************************** Douban Books v2 (4, 2, 1) ******************************
Found 0 results
Downloading from Douban Books v2 took 4.281980037689209
start get metadata from douban...
{'isbn': '9781505658200', 'amazon': '1505658209', 'ff': 'a/daniel-arenson/alien-hunters', 'fictiondb': 'daniel-arenson~alien-hunters~1197738~b', 'goodreads': '24533763', 'kobo': 'alien-hunters'}
9781505658200
http://douban.com/isbn/9781505658200
Plugin Douban Books v2 failed
Traceback (most recent call last):
File "E:\Development\GitHub\calibre\src\calibre\ebooks\metadata\sources\identify.py", line 47, in run
File "calibre_plugins.dummy2.__init__", line 91, in identify
File "calibre_plugins.dummy2.__init__", line 292, in get_book
File "calibre_plugins.dummy2.__init__", line 377, in __get_author
IndexError: list index out of range
********************************************************************************
The identify phase took 4.30 seconds
The longest time (4.281980) was taken by: Douban Books v2
Merging results from different sources
We have 0 merged results, merging took: 0.00 seconds