IDocs are failing with one or more of the below errors:

EA(390): Problem with SOAP class
Error passing data to port
Error during syntax check of IDoc (outbound)

Open the IDoc that is failing with this error in transaction WE05 and check if there are any syntax error under the Status records folder.

Double click in the EDI: Syntax error in IDoc (segment cannot be identified) item should reveal what is the data structure giving syntax errors.

After understanding what is the Data Structure, open the IDoc Basic Type, as example ARBCIG_ORDERS, in transaction WE30 and check if the segment is present in the correct structure of the standard basic type.

If the segment is not in the right order in the structure, any customizations in the ERP system that are changing the standard IDoc structure must be corrected by the owners of the customization.

Customizations in the IDoc layer in the ERP system.

SAP Integration Suite Managed Gateway > Managed Gateway for Business Network
SAP Integration Suite Managed Gateway > Managed Gateway for Business Network SCC > Managed Gateway for Buyer Business Network SCC
SAP Integration Suite Managed Gateway > Managed Gateway for Procurement
SAP Integration Suite Managed Gateway > Managed Gateway for Sourcing Integration > Managed Gateway for Sourcing - Contract Workspace