I’m sending an RPC to a netconf device, the rpc-reply has a typo in the xml (the open element is apppack-download-statu but the close one is apppack-download-statu), hence ODL is throwing a fatal error but the exception is probably not propagated properly, as the RESTCONF request hangs indefinitely.
See logs bellow:

2017-06-08 12:59:14,715 | TRACE | upCloseable-3-13 | NetconfDeviceCommunicator | 289 - org.opendaylight.netconf.sal-netconf-connector - 1.4.3.Boron-SR3 | Finished sending request <rpc message-id="m-1" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<request-appid-application-package-download/>
</rpc>

2017-06-08 12:59:14,730 | TRACE | NioProcessor-16 | AsyncSshHandlerReader | 124 - org.opendaylight.netconf.netty-util - 1.1.3.Boron-SR3 | Reading message on channel: ChannelSubsystem[id=0, recipient=0], message: <nc:rpc-reply xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0" xmlns:junos="http://xml.juniper.net/junos/15.1X49/junos" message-id="m-1" xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">

2017-06-08 12:59:14,896 | TRACE | NioProcessor-16 | AsyncSshHandlerReader | 124 - org.opendaylight.netconf.netty-util - 1.1.3.Boron-SR3 | Reading message on channel: ChannelSubsystem[id=0, recipient=0], message: <apppack-download-statu xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<apppack-download-status-detail>Please use command
"request services application-identification download status" to check download status
</apppack-download-status-detail>
</apppack-download-status>
</nc:rpc-reply>
]]>]]>

2017-06-08 12:59:14,896 | TRACE | upCloseable-3-13 | NetconfXMLToMessageDecoder | 124 - org.opendaylight.netconf.netty-util - 1.1.3.Boron-SR3 | Received to decode: 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
2017-06-08 12:59:14,896 | WARN | upCloseable-3-13 | NetconfXMLToMessageDecoder | 124 - org.opendaylight.netconf.netty-util - 1.1.3.Boron-SR3 | XML message with unwanted leading bytes detected. Discarded the 1 leading byte(s): '0a'
[Fatal Error] :2:25: Element type "apppack-download-status" must be followed by either attribute specifications, ">" or "/>".
2017-06-08 12:59:14,897 | WARN | upCloseable-3-13 | DefaultChannelPipeline | 77 - io.netty.common - 4.0.44.Final | An exceptionCaught() event was fired, and it reached at the tail of the pipeline. It usually means the last handler in the pipeline did not handle the exception.
io.netty.handler.codec.DecoderException: org.xml.sax.SAXParseException: Element type "apppack-download-status" must be followed by either attribute specifications, ">" or "/>".
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:442)[79:io.netty.codec:4.0.44.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:248)[79:io.netty.codec:4.0.44.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:357)[78:io.netty.transport:4.0.44.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:343)[78:io.netty.transport:4.0.44.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:336)[78:io.netty.transport:4.0.44.Final]
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:293)[79:io.netty.codec:4.0.44.Final]
at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:280)[79:io.netty.codec:4.0.44.Final]
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:396)[79:io.netty.codec:4.0.44.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:248)[79:io.netty.codec:4.0.44.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:357)[78:io.netty.transport:4.0.44.Final]
at io.netty.channel.AbstractChannelHandlerContext.access$600(AbstractChannelHandlerContext.java:35)[78:io.netty.transport:4.0.44.Final]
at io.netty.channel.AbstractChannelHandlerContext$7.run(AbstractChannelHandlerContext.java:348)[78:io.netty.transport:4.0.44.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:399)[77:io.netty.common:4.0.44.Final]
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:446)[78:io.netty.transport:4.0.44.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:131)[77:io.netty.common:4.0.44.Final]
at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:144)[77:io.netty.common:4.0.44.Final]
at java.lang.Thread.run(Thread.java:745)[:1.8.0_121]
Caused by: org.xml.sax.SAXParseException: Element type "apppack-download-status" must be followed by either attribute specifications, ">" or "/>".
at org.apache.xerces.parsers.DOMParser.parse(Unknown Source)[:]
at org.apache.xerces.jaxp.DocumentBuilderImpl.parse(Unknown Source)[:]
at javax.xml.parsers.DocumentBuilder.parse(Unknown Source)[:2.6.0]
at org.opendaylight.controller.config.util.xml.XmlUtil.readXmlToDocument(XmlUtil.java:108)[114:org.opendaylight.controller.config-util:0.5.3.Boron-SR3]
at org.opendaylight.netconf.nettyutil.handler.NetconfXMLToMessageDecoder.decode(NetconfXMLToMessageDecoder.java:70)[124:org.opendaylight.netconf.netty-util:1.1.3.Boron-SR3]
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:411)[79:io.netty.codec:4.0.44.Final]
... 16 more