have your images / files you try to upload any kind of special chars in their filenames? For Example german umlauts or other global-kind special chars.
Are your filenames displayed completely (no stripped names) in the media-kind-area nof the backend?
Background: I'm currently migrating some ezPublish's v2 and v3 and encountered several non-utf8-string-failures (whatsoever) according to filenames and file-contents (when indexing with ezfind. but thats another thread).