After upgrading Notes Client on Linux to 8.5.3 (it is in fact related to update of embedded sametime to 8.5.1) I started to get an error message "Unable to initialize Computer, and it can't be used at this time. Processed unsuccessful response: null for: REGISTER..." and then there were some details about the connection. Quick Goggle search helped: How to suppress SIP registration failures in the Sametime 8.5x client
Another variant of this error is on the picture. Both errors stopped appearing after adding the
com.ibm.collaboration.realtime.telephony.softphone/suppress.failed.sip.registration=true line to
It should work on Windows too, but I don't know if this error happens there as well. Let me know if it does and whether this "fix" works there.