Thanks. I reverted and tried again (with the app pool user set to NetworkService) and it worked this time, thankfully.
As it worked, I can't give you the exact error message but it did say something like "SqlDataProvider: Object already exists" while running the DB script in Install.aspx - that's as much as I can give you, I'm afraid.
You may be able to replicate the issue by trying to upgrade from 1.4.256 to 1.5.256 with an application pool user of "ApplicationPoolIdentity" in IIS 7?
As it worked, I can't give you the exact error message but it did say something like "SqlDataProvider: Object already exists" while running the DB script in Install.aspx - that's as much as I can give you, I'm afraid.
You may be able to replicate the issue by trying to upgrade from 1.4.256 to 1.5.256 with an application pool user of "ApplicationPoolIdentity" in IIS 7?