"We probably have chosen a good solution"? This seems just as good as "We have probably..."
Yeah, I believe so. Added now.
why not : probably we have chosen a good solution
That doesn't seem to be a good word order in English.
Actually, that is perfectly normal English. Regularly we will start a sentence with an adverb -- e.g., these sentences... :-)
Technically, there should be a comma after the adverb "regularly" though. (oops, I did it again) :-)
The problem with commas is that firstly, Duolingo ignores them, and secondly - it always seems to me like it's a bit different sentence if English introduces a comma that wasn't there in Polish.
But anyway, we decided to accept it here.