Hi Bettah , the error reads "400 - Bad request". The most likely cause is that the invoice data submitted to MYOB did not meet the API requirements. To resolve this, MYOB would need to see the header and body content of the call to the API. This is not something that you would have access to, in fact it may not even be retained on your computer. But Syrinx support may have access to these records if they are a cloud solution.
Each vendor's API is different, so knowing that QuickBooks and Xero transfers both work is not going to help.
Looking at the invoice in Syrinx, does it look any different to others that have gone through?
Do you know why MYOB has been unresponsive to Syrinx support? MYOB API team do require support requests to be submitted in a particular format - see https://apisupport.myob.com/hc/en-us/requests/new?ticket_form_id=215283.
Have you also received different errors?