Resolved Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8mb4_general_ci,COERCIBLE) for operation '='

Hello,
Take a look at Deal with different collations in MySQL document and see if it can solve the error with solutions given in the document.

many thanks for you help.

How can I find what table have the problem?

They are a lot of virtuemart tables...

take a look at the product table, everywhere is utf8_general_ci

Hello,
You need to check the tables which are related to your import. Since you are running VirtueMart Product import, check the following tables

#__virtuemart_products

#__virtuemart_products_fr_fr

#___virtuemart_categories

#__virtuemart_categories_fr_fr

#___virtuemart_manufacturers

#__virtuemart_manufacturers_fr_fr

#__virtuemart_medias

#__virtuemart_product_customfields

#__virtuemart_product_manufacturers

#__virtuemart_product_medias

#__virtuemart_product_categories

Also make sure the import file you are using is UTF-8 encoded, that could be one more reason for the error. This is also explained in the document i linked previously.

I notice, some of them are utf8_general_ci and others are blank, without anything. Is it ok?

Hello,
Yes that is ok. The collation is set to utf8_general_ci for fields which are defined as varchar or text. It will be blank for other field types like int, decimal or datetime. You need to check if collation is set different for any of these text fields defined as varchar or text.