Most likely cause is that you have two installation of OMNeT++ on the machine and your system path may contains one (or both) of it. Once you try to start one it will load DLLs from the other installation as it is earlier in the path.
-> Check your system path and remove everything that is OMNeT++ related.
Generally OMNeT++ never touches anything that is global (including your system path) so you have probably added that PATH element manually sometime in the past.