Amazon Metadata Plugin Not Used When "metadata-sources-cache.json" Exists

[GCW]

Calibre-version: 2.81 x64 (2.80 is not affected)
OS: Win 10 x64

When downloading metadata for an item, the Amazon-plugin is only used as long as no metadata-sources-cache.json file exists in the configuration folder.

Log excerpt:

without file present:
Using plugins: Goodreads (1, 1, 12), Google (1, 0, 0), Amazon.com (1, 0, 0), ISBNDB (1, 0, 0)

with file present:
Using plugins: Goodreads (1, 1, 12), Google (1, 0, 0), ISBNDB (1, 0, 0)

[Kovid Goyal (kovid)]

I cannot replicate this. Run calibre in debug mode and post the log when
you download metadata.

 status incomplete

[Kovid Goyal (kovid)]

Nevermind, I think I know what's causing it.

[Kovid Goyal (kovid)]

Fixed in branch master. The fix will be in the next release. calibre is usually released every alternate Friday.

 status fixreleased

[Kovid Goyal (kovid)]

Just delete the metadata_sources_cache.json file and you will be fine --
I have changed the minimum required version for the updated amazon
plugin to be 2.82 so it will not be used again until the next calibre
release.