After searching the web for a while I've seen a few posts with my exact problem and no responses at all.
After installing the latest Cumulative Update (CU5) for BizTalk 2020 we have gotten the following error for our SFTP-adapter.
Application: BTSNTSvc64.exe
Framework Version: v4.0.30319
Description: The process was terminated due to an unhandled exception.
Exception Info: System.MissingMethodException
at Microsoft.BizTalk.Adapter.Sftp.SftpFileReceiver.PollSftpServer()
at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
at System.Threading.ThreadHelper.ThreadStart()"
After some searching on the web several other people have the same problem and it seems to be some sort of WinSCP mismatch.
I have tried version 6.3.3 – 6.3.5 with no luck. Also tried WinSCP-6.3.5-Automation.zip-pack and all the different versions of WinSCP there (netstandard2.0 and net4.0) (I swapped both the DLL in the install directly and the one that is put side by side of the Microsoft.BizTalk.Adapter.Sftp.dll in the GAC.
Except for the few hits on other people with the same error I really don't know where to turn. Don't know if the error helps in any way but its all I'm getting from the Windows Logs