Wireshark 2.6 could not decode diameter messages

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Is the trace taken natively (e.g., with tcpdump or Wireshark) or was it created by some other method?

I suspect it was created by another method and whatever is creating the trace is putting fake TCP headers that don't have correct sequence numbers.

Regardless you should probably be able to avoid the problem by disabling the TCP preference for analyzing sequence numbers.

ps. if you open a bug please be sure to post a link to it here.

Raise a bug report with a sample trace. It hard to speculate what the problem might be. Possibly duplicate messages you could try to filter those out with editcap.