As I know InMobi is an ad network and MobFox is mediator, that supports AdMob and Inmobi as well. So if you want to use AdMob mediation, you're likely to exclude MobFox SDK, because they can conflict with each other. My guess is that you're trying to implement several SDK with same libs of networks they support, this leads to the error. I'd recommend to contact the support of mediator you're gonna use, because they should know better what's wrong.
I also tried MobFox and frankly, it left much to be desired. The integration was rather problematic and support didn't reply at all. It was about a year ago, hopefully the situation already changed =)
As for mediators, i've tried several on my android game. Mobfox performed not quite well for me, big problems with fillrate and settings. In my case Appodeal is the best solution for now, explained why in other threads. Also tried fyber, which is much better then mobfox, still it has complicated settings and integration, main con for me was the fact that i have to set each ad network manually in fyber's mediation. Guys from appodeal do it automatically, i only integrated their sdk and linked up my admob account. I didn't add any extra sdks of any networks they support (among them admob, inmobi, facebook, chartboost, unity, vungle and many others). Just check if you really need full mobfox and inmobi sdks for using admob mediation.
btw how long do you have that problem; what ad format do you use and which eCPMs do u have in AdMob and set for InMobi and MobFOx?
