To solve this problem we advise to check the synonyms configuration on your Magento extension:
Stores > Configuration > Algolia Search > Synonyms
- There shouldn't be an empty field on your synonyms
- There shouldn't be any field that ends with a comma
In case there are any of these problems on your configuration, you can try fixing this and then try reindexing again.
In case this doesn't solve the issue you can create a Support ticket
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