I have found, if I disable the indexing on the title field, the process runs through without errors. I believe there might be some sort of issue with one of the filters hooked onto the 'the_title' WordPress filter, which is executed before the plugin indexes the title.
Currently, the title indexing is disabled, but there might be a way to resolve it via a custom code:
1. Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case!
2. Then try to re-enable this option and make a new index: https://i.imgur.com/IwmGm92.png
This may not work though, but let me know.Best,
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