This is somehow related to the availability of options for products. I had the same error and viewing the profiler in the Doctrine information was an error for two objects that were stored in the database. In particular, it could not display Sylius \ Bundle \ AssortmentBundle \ Entity \ CustomizableProduct or Sylius \ Bundle \ AssortmentBundle \ Entity \ Variant \ Variant.
To fix this, I had to generate variants of the product, which is located on the product details page in the admin area. Having no options to add to the basket, he could not select a specific type of product to add to the basket.
It seems to me that this should lead to a better error, especially when starting the application in dev mode.
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