I switched the export output format to MPG and have recreated a few exports using the same project (identical project, absolutely NOTHING changed from one run to the next one). The problem occured in each "export", but on differents locations. It looks as if the problem is caused by reasons external to the VIDEOPAD project (e.g. synchronization between threads that are depending on each other's timely completion; CPU's switching into "turbo"; slow down of certain processor clocks because of high temperature; interference by other, non-VIDEOPAD, active application; ...).
Then I did remove all crossfades between the stills and the small videos (all stills and videos are in video track 1). But there still are a couple of cross fades transitions in video track 2, in the "text" images that overlay some stills and videos in track 1. After I removed all crossfades in videotrack 1 the problem seems to have disappeared. Of course the exported "film" does not look very good without the fading (to switch from one image (or video) to another, the show "jumps" now rather than performing a smooth fade-out/fade-in transition). I could live with this for the moment (as a "workaround), but I surely would like to be able to add the crossfade transitions again before I will make the final version of my show.
By the way, my PC has an Intel I7 4790 (no K) processor and 16 GB of memory. It has no graphical processor apart from the GPU built in into the CPU chip, because 95% of the time I use it for applications that do not require more graphical power (Office-like apps, mail, surfing, no games, ...). It has a SATA III SSD (system and apps, page, hibernate, ...) and a 3TB 7200 RPMs HD (contains ALL data, including temp files). I guess that should be sufficient "power" to run VIDEOPAD now and then?
Soon I want to create some more videos using VIDEOPAD. I understand that the problem could well re-appear if I add functions that require (or overload) certain resources? Is there any VIDEOPAD functionality that I should avoid until the problem is resolved?