I have noticed strange behaviour while I made backup of my configuration with SSTP server and certificates.
After restoring it on second device SSTP configuration was off and certificates were not migrated
There was a bug with the certificates that got resolved in 5.14. Maybe try a newer version ?
Myself am encountering a number of issues running SSTP server using 5.15 so I am advising you to steer clear from 5.15.