This occurs on a Raspberry Pi 3 Model B+ with a manual install on openHAB 2.4. The errors I receive in /var/log/openhab2/openhab.log lead me to believe it is a relatively simple fix in that when the PIM receives a UPB message that the byte length of the message is verified with a normal UPB message. See example error below
2019-10-12 11:42:09.113 [ERROR] [nhab.binding.upb.internal.UPBMessage] - Attempted to parse invalid message: APP7
java.lang.IllegalArgumentException: contains illegal character for hexBinary: P7
at javax.xml.bind.DatatypeConverterImpl.parseHexBinary(DatatypeConverterImpl.java:451) ~[?:?]
at javax.xml.bind.DatatypeConverter.parseHexBinary(DatatypeConverter.java:357) ~[?:?]
at org.openhab.binding.upb.internal.UPBMessage.fromString(UPBMessage.java:137) [230:org.openhab.binding.upb:1.13.0]
at org.openhab.binding.upb.internal.UPBReader.interpretBuffer(UPBReader.java:157) [230:org.openhab.binding.upb:1.13.0]
at org.openhab.binding.upb.internal.UPBReader.addData(UPBReader.java:110) [230:org.openhab.binding.upb:1.13.0]
at org.openhab.binding.upb.internal.UPBReader.run(UPBReader.java:180) [230:org.openhab.binding.upb:1.13.0]
at java.lang.Thread.run(Thread.java:748) [?:?]