first of all, thanks a lot for your quick reply!
I have to apologize for not being precise in my post above.
"IP Webcam" - check
Kinovea 0.8.26 x64 - check
Now the settings I used today:
IP Webcam video settings - left untouched, .mkv format is pre-defined.
Kinovea settings - Capture > General > Display synchronization > "Camera frame" AND Capture > General > Video format: MKV (not sure wether it was pre-defined here).
Used WiFi hotspot on the phone, connected the PC to it.
Followed the same steps you described in your post - it worked fine for both now, browser & Kinovea.
The only difference to my previous trial is... I closed the browser window this time before opening stream in Kinovea.
Thanks a lot, once again!