Solved.
If someone has a problem after importing images with this tool, it is caused by inserting the value "" into the file_lang column in the virtuemart_medias table.
Repair:
UPDATE j3_virtuemart_medias SET file_lang = NULL;
Because the quotation marks are meaninglessly inserted into multiple columns, I added another fix:
UPDATE j3_virtuemart_medias SET file_params = NULL;
UPDATE j3_virtuemart_medias SET file_class = NULL;
UPDATE j3_virtuemart_medias SET file_meta = NULL;
UPDATE j3_virtuemart_medias SET file_description = NULL;
I'm not a programmer and I helped myself with the MySQL manual. He would definitely come up with a nicer notation.