Karl I just wanted to give you an update. My hosting provider just responded and informed me that exec() function is actually enabled. I should of thought to test this earlier with other video files from various cameras and resolutions as it seems to be related to the video file itself.
I just uploaded a video file from a GoPro Hero 8 with 1080p resolution and the thumbnail preview appeared! So it seems uploading 4K resolution video file from my Sony ZV-1 is what causes the broken thumbnail image to appear. All my video files have been MP4 format and various sizes that range from 50MB to 300MB. It seems to make no difference on the size of my 4K video file, it has trouble generating the video preview thumbnail.
Thank you for your prompt response!