(Sorry it took me a while to get back)
Something that I might not have explained clearly is that this issue is coming up with ALL of these videos that last more than 25 seconds. So it is not related to flashes or other things. It is related specifically to video "length".
I have been able to somehow circumvent the problem, but still some questions linger.
To avoid the problem, I am opening the video clips in QuickTime and exporting them either with H.264 720p or with HEVC 1080p (the original clips are 1080p). In either of these situations, iMovie is fully reading the clips. Looking at all the clips' information, I notice that iMovie is reading files whose format is clearly stated as HD (1-1-1). If I don't see this information on a clip that lasts longer than 25 sec, iMovie will show the black screen at 25 sec. time.
For reasons unbeknownst to me, when I try to export the clips with H.264 1080p, the HD (1-1-1) categorization does not come out and they fail on iMovie. So for now I'm left with two choices: quickly export them with H.264 720p or slowly export them with HEVC 1080p.
I have not found a way to make QuickTime export with H.264 1080p HD (1-1-1). Do you know how to do it?
I know now what works and what not. But I don't understand why. Any clue with what I've mentioned here?
Thank you very much!