LoadPackage fails from .NET with Integration Services class cannot be found

I know this has been asked many times before but I haven't found a solution to my specific problem.

My setup is a newly installed Windows 10 Enterprise fully updated with SQL Server 2017 incl Integration Service and Analysis Service. I've installed latest SSDT as well. I've also installed client tools SDK from the SQL Server 2017 installation. Visual Studio 2017 is installed as well.

My ASP.NET application 'should be' running SSIS packages, but it fails when loading the package using LoadPackage(packageName, null). The error is "an integration services class cannot be found etc etc.....". Just to repeat - Integration Service 2017 and Client Tools SDK are installed. My application has been working on previous SQL Server versions - the most recent version is with SQL Server 2014, but I've upgraded my application from SQL Server 2008 to 2008R2 to 2012 to 2014 and I've never met problems before.

What is different from other posts I can find is that I do not deploy the SSIS package to a separate web server (well, I do and that doesn't work either but a first step is to solve it on my dev machine). I have created an empty SSIS package in VS2017 to ensure it is not the content of the package that causes the error. The SSIS package is developed on the same machine, and I can run the package with both 32 and 64 bit versions of DTExec.exe successfully.

I have updated any references in my ASP.NET application to ManagedDTS.dll version 14.0. I've updated my ASP.NET application to .NET 4.7 as part of this update to SQL Server 2017.

As I can easily search and find many posts about similar problems with lots of solution suggestions, I hope someone will reply to this with solution suggestions rather than links to 'copies' of my question, as chances are I've already read those and tried all suggested solutions.

Thanks in advance...

UPDATE: First of all, creating a new project and loading a package works - so I figured that I might be facing a problem with the code after updating from .NET 3.5 to 4.7. I started commenting code and found that if I don't do impersonation before calling LoadPackage, it actually works. Based on some other issues I've had recently with the July updates from Microsoft, I have removed one of the updates and I now get a different error - with the impersonation code enabled. The current error is one I've seen many other people have so I'll dig into that - it is: "The package failed to load due to error 0x80131534 \"(null)\". This occurs when CPackage::LoadFromXML fails.\r\n". I don't really understand if this should be caused by a mismatch between versions, but I'll update as I find out more. If anyone has good suggestions, I'd be happy to hear them :)