Thanks for the answer. It seems that the problem has disappeared. I had the chance to test the SupportAssist routine a second time and this time it continued until the end without any further error. If the problem occurs again, I will test your proposed solution. 
SupportAssist 3.5.0 hangs while checking unwanted system files. It is step 2 from 5 in total of the routine. The only change since last action of this routine was the installation of the Avira antivirus program. This is only for information, but if you know a practible solution to avoid this, I would be glad.
SupportAssist 3.5.0 hangs while checking unwanted system files