I am also using Serviio and have no issues playing back files, I find that only MKV files have an issue with fast forward and rewind on the TV. I usually just convert them to mp4 with Handbrake and they play fine from that point.
For all you LC-60LE640U owners there is a new firmware for the TV out today.
This firmware improves the performance of DLNA playback, as well as contains improvements to the menu and Demo mode settings
This firmware also contains all previous fixes and enhancements.
Just updated it noticed a new option in the picture settings called Black Level, unfortunately though it seems to be grayed out.