The ielib64.dll is not in the distribution. But ievision64.dll is in the folder where the exe of the programm is located. I thaught that ievision64.dll contains also ielib64.dll!?
That is correct. If you have ievision64.dll, you don't need ielib64.dll. The ievision dll's come with IEVision, whereas the ielib dlls are included with ImageEn (in the \DLL\ folder where you installed ImageEn).
IEVision64.dll should be fully thread-safe, so it is likely to be some other issue (possibly not related to ImageEn).
You need to simplify the problem. Try to create a demo with the bare minimum code needed to reproduce the issue.