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The certificate error itself will not generate traffic, however, this error is potentially due to incoming traffic trying to reach the router using ssh or https and those will generate traffic.
If the device has a public IP address, the above is likely what is causing this and improving security on the device, such as configuring and enabling the firewall would help, see our TransPort security lock down document here: ftp://ftp1.digi.com/support/documentation/AN_018_TransPort_Security_Lockdown.pdf
Without seeing complete logs from the device, it will be hard to comment further.
For any further help or investigation on that issue, please review our support options on our website: https://www.digi.com/support and mail our support team at firstname.lastname@example.org.
Digi Technical Support Engineer