You will see this error for the customers using RECKON. When you see this error: Reckon GetDocumentInvoiceInformation' - 'The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters. '

This means the cache file is corrupted. You can access this file under Supplier folder. 

Path C:\Program Files (x86)\The Order Exchange\The Order Exchange\SUPPLIER FOLDER\Cache\Reckon. In order to access this folder>go to the order exchange>cog>open>supplier folder. You will access the above path. 

You will see a XML file under this path. Cut that file & paste it somewhere else (just so you have a backup).

Now, go back to the order exchange and process the orders. It will work.