KingsGroup.IT, since your original question isn't resolved yet, you could try to do the following to find out what Nicehash is doing differently:
Start mining and get a tool like Process Explorer (it's from a Microsoft MVP so it can be trusted if you have second thoughts) and rightclick the lolminer process and select Properties. Under "Command line" you should see the full config they used to start mining. If they use a config file, track it down with its path. I dunno whether Nicehash does this, if it runs lolMiner with admin privileges you need to run Process Explorer with admin privileges as well to get this information.

Probably not related, but I found when mining Beam on a R9 290 on Windows 10, it makes a difference to set lolMiner to enable Compute Mode. I didn't observe any influence of compute mode on mining Zhash on the 290 or the 7970 though so I doubt that is it.

I still hope lolliedieb will look into what I posted earlier here https://bitcointalk.org/index.php?topic=4724735.msg56572776#msg56572776, would be really cool to be able to mine Beam on Tahiti cards.