Eva:

1) Was BR updated between the time the import worked and when it stopped working?
2) When you say "identical transaction" do you mean an actual file that was successfully processed from the period when it was working?
3) If not, do you have an actual file from that period and what happens if you try to import it?
4) Could there be more than 1 import template?
5) Does the new file match the olders one(s) in terms of number of fields and types of delimiters (quotes and commas)?
6) Is there a header row now when there wasn't before?
7) Did the cheques have negative signs before?
8) Does the template have the same number of fields in the correct order?

When all else fails, break the file down to a single row and try importing that. Manipulate the template until it works.

Or, as an alternative to this whole process, does the bank's web-site offer other download formats? If it offers a Quickbooks format, use that and then you don't need an import template at all.