I ran into this problem a while ago at a client. They upgraded from Visual Studio 2013 to 2015 and the SSAS processing tasks started to error out immediately. The solution turned out to be setting the TargetSeverVersion to anything but SQL Server 2016. In this case, it was set to 2014 and that fixed the error.
Recently I ran into this post https://twitter.com/SQLKohai/status/994335086425399297 by Matt Cushing (@SQLKohai) and decided to dig in more. Initially when I tested it, all was working fine. After I installed SSDT 2015 to test, I started getting the same error in SSDT 2017. I played around with a DLL and got SSDT 2017 to work with all TargetVersionServers again. At the end I managed to break it again after I went through an uninstall and reinstall of all versions of SSDT. The reason I did the reinstall of SSDT was that I thought I might have had a broken registry entry that I was hoping the installation would fix. This did not work!
Read on for the solution and a detailed dive into the problem.