TeamViewer Free - connection fail temp

I've been using TeamViewer Free for some years with a friend and 99% of the time no hiccups, we're both using Windows 7.

He asked me to help his sister in Arizona, she is using Windows 10. As usual connection fast and audio also perfect. I can access anything, generate new folders, install new progs, etc.

Now the hiccup - the moment I start a newly installed prog TeamViewer pops-up with "connection temporarily lost" or something similar. We still have audio but my mouse out of action. I can tell her what and where to select but it is extremely cumbersome.

The moment she closes out of that program my mouse back as normal.

Seems to me, only a deduction, I do not have permission on her PC at that stage, is there an option I am missing.

I will be very thankful if someone can give advice.

Regards