Maybe there are plugins from other extras that run on the events “OnFileManagerBeforeUpload”, “OnFileManagerUpload” or “OnFileManagerFileRename” and possibly interfere?
Under cogwheel-icon → “System Settings” there is a tab “System Events” to check that. Use the event name in the search field, then check the column “Plugins”.
Does it make any difference if you change the value of the system setting upload_translit?