The message appeared when trying to edit a product. However, I found on the internet that the issue could be caused by the differences in the collation of the tables/columns. Finally, I changed all the charset and collations of all the tables of Sellacious, for example:
ALTER TABLE `r9fim_sellacious_addresses` CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
And the problem was solved. However, I would like to know if this is right for the performance, etc.