Import Wizard giving Assertion Error for some files

Hi all,

We use the import wizard to import contacts and accounts and just started having a problem with it.

When we choose which file and click next to map the fields, we receive this error:

It occurs for different files that have a lot of data rows and also occurs for files that have only a handful of data rows.

We have checked there are no empty columns in the middle of the data and it is saved in the correct .csv format.

Thanks in advance for any help