We see the reconnect warning occasionally in Desktop but not necessarily with SAP BW. We've been connecting Tableau to BW for 5 years using SAP Logon 7.3. We are currently using Tableau 2018.1 but I just checked that 2018.3 seems to connect reliably the same way. We always pull extracts from BW, we never use live data from BW.
I see the same error. Have you been able to solve the problem?