Multilingual site title
Well, I have experienced a similar problem when manipulating with more language versions. In the config database table, I had two rows with the name system.site - one of them had a collection = "language.sk" and the other (full record) had an empty collection and in the data column langcode SK. So, I think the interface was confused, because there were two rows for one language. However, I do not know how the database got into this state.  

So I have deleted the row with collection = "language.sk", cleared the cache and now you can change source site name and also create translations - it started to work. Please try this and let me now, if that works.