Symphony.

I believe there is an error on line 397 in file field.youtube_videos.php. It was orginally:

Symphony::Database()->update($data, "sym_entries_data_{$field_id}", "entry_id={$entry_id}");

But this only works if your Symphony is configured to use sym_ as table prefix. Changing it to sym_ fixed the problem for me as sym_ is replaced with the set prefix.