in 0.9 we've changed ofMutex from using poco fastmutex to std::mutex and ofScopedLock from poco scopedlock to std::unique_lock ofMutex and ofScopedLock should be deprecated but there's no way to do it yet in visual studio and some old versions of gcc and clang so we've left them there by now.
It seems you might be using Poco::FastMutex directly in your code, which will fail when combined with unique_lock, you should change any appeareance of that to std::mutex