Trying to patch last of 17 sites (10.4.2 to 10.6.1). For some reason I can't find a local copy or git copy of it at 10.4.2, even though the live site is 10.4.2 so obviously at some point it did exist. So not sure what happened. Anyway, version I have is at 10.1.1 but database seems to be 10.4.2. It won't let me run locally or upgrade, keeps giving >

Umbraco.Cms.Core.Exceptions.BootFailedException: An error occurred while running the unattended upgrade.
The database configuration failed with the following message: The migration plan does not support migrating from state "{3F5D492A-A3DB-43F9-A73E-9FEE3B180E6C}".
Please check log file for additional information (can be found in '~/umbraco/Logs')

Tried upgrading to 10.4.2, then tried going back to 10.2 but keep getting same - is there anyway around it? If I set up a new database for it to link to and set up at whatever the site is at at this stage, would that work? Then I could switch databases?

Yeah, been having some of this here as well. Takeaway is to upgrade often, upgrade early, make sure to actually deploy all upgrades, always upgrade to the latest minor before upgrading to a new major, and so on.