As @simonthesorcerer wrote the problem are the data in the database! In our case the migration of Magento from 2.1.15 to 2.3.3 thrown an exception in the field value of the quote_item_option table.
The error was the wrong lenght of a string mentioned in the serialized data:
a:8:{s:4:"uenc";s:60:"aHR0cDovL3d3dy5zbGlkZXNob3BwaW5nLml0L2l0YS9jaW4tY2luLmh0bWw,";s:7:"product";s:4:"1078";s:28:"selected_configurable_option";s:0:"";s:15:"related_product";s:0:"";s:15:"super_attribute";a:1:{i:90;s:3:"118";}s:3:"qty";s:1:"1";s:10:"action_url";s:131:"http://mysite.local/ita/checkout/cart/add/uenc/aHR0cDovL3d3dy5zbGlkZXNob3BwaW5nLml0L2l0YS9jaW4tY2luLmh0bWw%2C/product/1078/";s:4:"ajax";s:1:"1";}
and the correct one is
a:8:{s:4:"uenc";s:60:"aHR0cDovL3d3dy5zbGlkZXNob3BwaW5nLml0L2l0YS9jaW4tY2luLmh0bWw,";s:7:"product";s:4:"1078";s:28:"selected_configurable_option";s:0:"";s:15:"related_product";s:0:"";s:15:"super_attribute";a:1:{i:90;s:3:"118";}s:3:"qty";s:1:"1";s:10:"action_url";s:130:"http://mysite.local/ita/checkout/cart/add/uenc/aHR0cDovL3d3dy5zbGlkZXNob3BwaW5nLml0L2l0YS9jaW4tY2luLmh0bWw%2C/product/1078/";s:4:"ajax";s:1:"1";}
the "action_url" data has been set with 131 characters instead of 130!! fixing this the migration works!
Thanks to all who help me with stackexchange!