When using redirect payments, this error occurs:
BackUrl too long : length xxx MAX:200
The Ingenico platform allows only up to 200 characters for the Back URL.
If the Hash algorithm is set to SHA-512, the value of the parameter SHASIGN already takes up 128 characters.
If in addition to that the URL Rewrites are not enabled on the server, the string "index.php" will be added to the URL, leaving even less space for the rest (base URL, other GET parameters).
In total, this can make the URL longer that 200 characters.
Set the Hash algorithm in Magento and in the Ingenico backoffice to SHA-256. This way, the SHASIGN will only be 64 characters long, which leaves enough room for the other parts of the URL.
This behavior may be improved in future module versions.