When a transaction in the eVA/AIS integration goes to error status in AIS, Integration Broker (IB) stops processing all other messages in that queue until the erroneous message is cleared.
The errors are either in Subscription Contracts, due to special characters in the XML message, or in Publication Contracts, because DGS servers are temporarily unresponsive.
Subscription Contracts
If special characters exist in the XML, you will find a transaction in the Subscription Contracts of 0 bytes (a null message). The only resolution is to cancel the message.
Navigate to:
Main Menu > PeopleTools > Integration Broker > Service Operations Monitor > Monitoring > Asynchronous Services
Ensure that the Queue Level is set to Sub Contract and that the From and To Dates and Times are sufficient to cover the suspected time of the error. Refresh.
When an incoming eVA message is in error, the number will show under the Error column next to the Service Operation VX_EVA_IN_AS. Click on the number to see the message in error.
Clicking Details, then Error, will show you the error message. In the case of special characters in the XML, the message will be as follows: 
The Uncompressed Data Length will also be 0.
Cancel the message.
After cancelling the message, monitor the queue until it is clear to ensure that no others go to error status.
eVA sends the supplier integration message with every Requisition and Purchase Order. If the supplier message is the one containing a special character, it will be sent again soon after the first is cleared, so check the queue again several minutes later. If the supplier message was at fault, the Requisition transaction may be denied because no supplier exists. SSC monitors the integration queue and will take action if this is the case.
If the Requisition or Purchase Order XML is at fault, the transaction will have to be edited and sent again by DGS, or the XML provided by DGS and manually submitted in AIS after removing the special characters.
Publication Contracts
Less frequently, the DGS server is temporarily unresponsive, and an Approve/Deny response will go to error in Pub Contracts.
Change the Queue Level to Pub Contracts and look for a message in error in the VX_EVA_REPLY Service Operation.
Click on the number in the error column to see the transaction in error.
Select the message and Resubmit. Monitor to ensure that the message transmits this time. If it fails again there may be a problem with the IB configuration or DGS may have server problems in their side. In that case, reach out to DGS for advice.