Unable to fetch a list of attachment IDs to process from the WordPress REST API.

I am attempting to regenerate thumbnails for two new sizes that I created but keep getting this failure notice at 0% – before the process even starts:

Unable to fetch a list of attachment IDs to process from the WordPress REST API. You can check your browser’s console for details.

When I check the console this is what I’m seeing:

[Log] JQMIGRATE: Migrate is installed, version 1.4.1 (load-scripts.php, line 9)
[Log] Regenerate Thumbnails: Error getting the total attachment count. (3) (build.js, line 1)
Object
"parsererror"
SyntaxError: JSON Parse error: Unrecognized token '<'
[Log] Regenerate Thumbnails: Error getting the total featured images count. (3) (build.js, line 1)
Object
"parsererror"
SyntaxError: JSON Parse error: Unrecognized token '<'
[Log] Regenerate Thumbnails: Error getting a chunk of thumbnail IDs to process. (3) (build.js, line 1)
Object
"parsererror"
SyntaxError: JSON Parse error: Unrecognized token '<'
[Log] Regenerate Thumbnails: Error getting a chunk of thumbnail IDs to process. (3) (build.js, line 1)
Object
"parsererror"
SyntaxError: JSON Parse error: Unrecognized token '<'
[Log] Regenerate Thumbnails: Error getting a chunk of thumbnail IDs to process. (3) (build.js, line 1)
Object
"parsererror"
SyntaxError: JSON Parse error: Unrecognized token '<'

Do you have any clues as to why this is happening?

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