The numThreads option is intended to speed up restoration by performing some processing in parallel using multiple threads. The main operations that benefit are (a) restoring the data for large tables (many rows) and (b) index creation. When operating with multiple threads dependency checking is necessary to ensure that one thread is not performign an operation that may depend on an object that has not yet been created by another thread. This can slow things down in cases where there are not many large tables with many indexes.
The error that you are seeing would appear to be a dependency checking bug. Please file an SR with all the necessary details so we can investigate.