Previously we have had sites report that the barcode will not scan if the background colour on the email confirmations is a dark colour.
In the instance logged the background was dark grey.
If there is a colour set here:
It will be overridden by whatever is set here:
(For reference #ffffff is white, screenshot taken after this was updated)
Modify the colour (if you are unsure of the hex, you can use a colour chart online which will give you the correct value).
Ensure you save the changes.