After CS Cart upgraded to jQuert 3.5.1, that is already obsolete, they did not update core js files to replace ".size()" with "length", which caused for me that login popup when submitted with incorrect username/password reloaded the page instead of showing errors.
I am writting this here as I cannot start a bug tracker thread. Please fix this CS Cart as I am not happy about making changes to core .js files that will get overridden on upgrade. This bug will affect much more than just login popup.