Starting from FineReader Engine 12, FREngine object is no longer a singleton. If you load the Engine in a way, which allows multi-threading, then the number of simultaneously loaded Engine object is limited by your RAM size. Each FineReader Engine instance might take up to 350 Mb of memory + memory for the FRDocument object. Make sure that you have enough RAM for a few instances.
The full description of the way to use Engine in a multi-threaded application is described in Developer's Help, chapter "Guided Tour → Advanced Techniques → Programming Aspects → Different Ways to Load the Engine Object"