Exporting Videos not working anymore

Dear all,

I just started using Shotcut and am rather new to Video editing. I am putting together a video with inputs from several people, so using different formats (MP4 and MOV). There first few exports went well, but the last few times, it has not worked anymore. The exporting process is much slower, and when I open the file it only has sound, but no image.
This morning I went back to a previous project, exchanged some files and exported it again and it worked fine. But trying to export any other compilations still has not worked.

Any ideas what the reason could be? I urgently need to make progress with this project.
Thanks in advance for any hints…