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Problem:
The taxonomy "translation_priority" was missing on the client site.

Solution:
To fix this, you need to go to WPML > Languages.
There click on Add / Remove Language and click on the "Save" button without selecting anything else and wait for the Ajax process to complete.
Once this is done, please clear your browser cache and re-open WPML > String Translation page and filter by any domain that you wish.

Relevant Documentation:
https://wpml.org/errata/no-strings-in-wpml-string-translation-screen/