Unable to Create new Maintenance plan in SSMS 2017
I was just searching for this error as it just occurred on my instance as well. Although it wasn't identical, I wanted to put this out there if anyone else runs into the same problem when searching.

Server 2019
SQL Server Standard 2017 (14.0.2027.2)
SSMS 2018 (v.18.5.1)

Went to create a new maintenance plan (the first one on this server and named instance) and received a very similar error out of the gate: Exception has been thrown by the target of an invocation. (mscorlib) Invalid Class String.
I checked all components, etc and my fix was to install SSMS 2017 and use it rather than SSMS 2018. Works just fine now.