I was able to upgrade a site from 7.1.2 -> 7.3.2, 7.3.2 -> 8.0.3 and 8.0.3 -> 9.1.1. The site was running fine right after each upgrade. I keep each version on the server.
However, for version 8.0.3 and 9.1.1, these site continues to directs to the http://mysite/Install/UnderConstruction.htm. I normally see this error if the DB connection string is incorrect. I verified the connectivity on the server and it's fine.
Under Windows events, I found the followings. I'm not sure if it's related to the issue I have.
Process ID: 3388
Process name: w3wp.exe
Account name: IIS APPPOOL\somePool070302
Exception type: ConfigurationErrorsException
Exception message: Unsecured Passwords Format Detected. The Membership Provider that contains the unsecure passwords format is: AspNetSqlMembershipProvider. The obsoleted password format is: Encrypted. For more information, see https://go.microsoft.com/fwlink/?linkid=834784.