This problem affects module version 17.4.0 for Magento 1. It is resolved in later versions.
After entering the credit card number and placing the order, the message "Your payment details will be forwarded..." appears. However, nothing happens. The buyer is not redirected to the Ogone payment page.
Check if module version 17.4.0 for Magento 1 is installed. If yes, you are affected by a problem in the module.
This problem is resolved in newer module versions.
In the meantime, the following quick fix can be applied.
Open this file:
Go to line 984. It should look like this:
Change the line to this:
return (string) $returnValue;
Save the file, flush all Magento caches, and try again. The issue should be resolved.