Hi Marian, I've seen that happen once before and I think it was due to the resolution combined with the export settings.
Try exporting using different settings (on most of the 'file' export screens there is a "Dimensions" option - try setting this to 1920x1080, and if that doesn't help, try changing the resolution of the canvas to either 1920x1080 or 1280x720, then export that and see if that helps.
Marian, I just had this happen. My issue was being on a mac with OS 10.10. If this is you, you have to close absolutely everything then only open camtasia and render the video. I also had to change settings to "download" vs "steaming" to get it to work. I have camtasia on both PC and mac and this only happens on the Mac.