First, setup your Skype for business client to collect "full diagnostic".
Open your Skype for Business client, click on "Tools" and select "Options" from the drop-down menu.
The options-dialogue should open. In there, select "General" (usually at the top), then find "Logging in Skype for Business" and select "Full" from the drop down list. Like this:
If done correctly, it should look like this:
Now click "OK" and Exit Skype for Business (we need to clean out some files and we can't do that while SfB is running).
Now, open up "File explorer" and copy this in the address, then hit enter.
The contents of that folder looks something like this:
Now, select all and delete it. The reason for this is to not include lots of traffic that's not needed to troubleshoot the issue. Select all files and folders, right click and then select delete.
If you get warnings about files that can't be deleted, you can skip those.
At this point you can start up Skype for Business again and reproduce the problem. Skype for Business will now write full log sets to this folder.