Undertow: Missing Start Boundary, or boundary does not start on a new line

I'm sending a SOAP request message using SAAJ directly (SOAPConnection::call) and I'm getting back from the server a response that can't be processed by the SAAJ RI:

Caused by: com.sun.xml.internal.messaging.saaj.SOAPExceptionImpl: Unable to internalize message
at com.sun.xml.internal.messaging.saaj.soap.MessageImpl.init(MessageImpl.java:521)
at com.sun.xml.internal.messaging.saaj.soap.MessageImpl.<init>(MessageImpl.java:340)
at com.sun.xml.internal.messaging.saaj.soap.ver1_1.Message1_1Impl.<init>(Message1_1Impl.java:70)
at com.sun.xml.internal.messaging.saaj.soap.MessageFactoryImpl.createMessage(MessageFactoryImpl.java:88)
at org.jboss.wsf.stack.cxf.saaj.SOAPConnectionImpl.readSoapMessage(SOAPConnectionImpl.java:298)
... 28 more
Caused by: java.lang.Exception: Missing Start Boundary, or boundary does not start on a new line
at com.sun.xml.internal.messaging.saaj.packaging.mime.internet.BMMimeMultipart.getNextPart(BMMimeMultipart.java:240)
at com.sun.xml.internal.messaging.saaj.soap.MessageImpl.init(MessageImpl.java:466)
... 32 more