The issue isn't usually 'what is missing' but 'what do you have too many of'?
The search for files to attach algorithm has to search through ALL the videos/pics, and has a tendency to time out.
The better solution is to move the files out of the File Storage folder tree if you need to keep them.
There is a work-around, though.
ssh or use the console to get a command line.
use the command:
nano /data/etc/motioneye.conf
add the line
list_media_timeout_email 20
<ctrl> x y to save
then use the command:
reboot
and test. If 20 seconds still doesn't work, you can increase it to 30. I don't recommend any higher though. It would be better to 'clean up' the File Storage' directory tree.