Hello Timo,
Could you tell me what CMS and client versions are you using?
Perhaps more about your PC spec as well?
Do you have any additional software installed on that PC that could interfere (like Afterburner for example)?
After you update your client to 1.7.6 (if you haven’t already) please do have a look at this topic: Object reference not set to an instance of a object
and specifically Dan’s reply there:
and after you download that .exe please check how it will work and we’ll deal with other issues if they will persist.