For sometime I experienced problems with Skype.
These problems only occurred on my iMac.
Currently my iMac is running macOS Sierra.
I use Skype on a daily basis to conduct standup calls with remote Scrum teams and to contact suppliers.
I noticed that group audio/video calls were not working anymore. Only chats were working.
After googling around and not finding any solution I got the impression it had something to do with my user account and some certificate.
Today I noticed that my machine was slow and I stated up Activity monitor. I noticed a background proces that was running :
Surprised as I was that this was running I killed the process and googled for background info of this process.
I found that this process was part of OSX Server.
I remembered that some releases ago I installed OSX server to have a look at it. I did not like it and I do not have a need for the features offered so I deinstalled it with some instructions I found somewhere.
Probably this deinstall was not performed completely and some stuff was still active.
I found this support page to deinstall opendirectoryserver
It seems this corrected my Skype problems.