Indeed, original config file was missing port info.
rstudio@rstudio-quickstart:~$ cat /etc/rstudio-connect/rstudio-connect.gcfg
; RStudio Connect Quickstart configuration file
[Server]
SenderEmail = rstudio-connect@example.com
Address = http://localhost:5000/rsconnect/
PublicWarning = <b>WARNING: This is a Quickstart Server and using it in production is not supported</b>
LoggedInWarning = <b>WARNING: This is a Quickstart Server and using it in production is not supported</b>
ExpandedViewUI = true
DefaultContentListView = "expanded"
[HTTP]
Listen = :3939
NoWarning = true
[Authentication]
Provider = pam
Notice = "WARNING: This is a Quickstart Server and using it in production is not supported. Login with user/password rstudio/rstudio"
[Applications]
RConfigActive = quickstart
[Python]
Enabled = true
Executable = /opt/Python/3.7.1/bin/python3
rstudio@rstudio-quickstart:~$ cat /etc/rstudio-connect/rstudio-connect-migration.gcfg
cat: /etc/rstudio-connect/rstudio-connect-migration.gcfg: Permission denied
rstudio@rstudio-quickstart:~$ sudo cat /etc/rstudio-connect/rstudio-connect-migration.gcfg
; This is an automatically generated file and it should not be
; modified. Use the command `rscadmin configure --append-migration` to
; move any settings declared here to your main configuration file.
;
; The Configuration Migration section in the Configuration Appendix
; of the Admin Guide explains how to work with the migration file.
;
[Server]
EmailProvider = "smtp"
[SMTP]
Host = "localhost"
StartTLS = "never"
; End of the migration
Edited
rstudio@rstudio-quickstart:~$ sudo cat /etc/rstudio-connect/rstudio-connect-migration.gcfg
; This is an automatically generated file and it should not be
; modified. Use the command `rscadmin configure --append-migration` to
; move any settings declared here to your main configuration file.
;
; The Configuration Migration section in the Configuration Appendix
; of the Admin Guide explains how to work with the migration file.
;
[Server]
EmailProvider = "smtp"
[SMTP]
Host = "localhost"
Port = 25
StartTLS = "never"
; End of the migration
I left the /etc/rstudio-connect/rstudio-connect.gcfg as is.
Had to keep the StartTLS = "never" setting for this to work 