• An archaeological excavation (Main Subject)
  • at what might have been a workshop (Prepositional phrase – modifies the main subject) 
    • where statues were reproduced (Relative clause – modifies “workshop”)
• yielded 1,532 fragments of human figures, (Main Verb: “yielded”)
  • including 7 intact statues. (verb-ing modifier – modifies “fragments”)

A couple of points to understand before we look at the problem in the sentence:

1. “What might have been a workshop” is a noun clause. Noun clauses are clauses that act as nouns in a sentence. In this case, this noun clause acts as an object of the preposition “at”. Noun clauses are actually quite common in regular English, even though some people must have been turned off by this noun clause in the original sentence. Read more about noun clauses here. Also, start observing noun clauses even in CR and RC sentences. This will make you comfortable with them.
2. Comma+including is an exception to the normal comma+verb-ing rule. Comma+including can modify not only the preceding clause but also the preceding noun. In this case, comma+including modifies the preceding noun “fragments”. The modification is grammatically fine.

The sentence has no grammatical issue but one meaning issue: the fragments cannot include 7 ‘intact’ statues.