So I haven't experienced this issue myself yet, but it has been wildly reported that a recent auto-update of Windows 10 is breaking MJPEG streams for applications based on DirectShow. This impacts for example the Logitech cameras in Kinovea.
There is a long thread over at MS Dev forums and the issue is apparently breaking many high profile applications like Skype.
Basically they wanted to support for multiple applications to consume camera streams so they moved the decoding stage upstream in the pipeline and removed access to the compressed stream.
They are working on a fix which should be pushed through auto-update in September.