More details needed. What exactly are you trying to do and how are you doing it, step by step ?
Please open Tools -> Messages (set Verbosity to 2) before you start the conversion, keep the messages window open and then paste the full resulting log here or on Pastebin.com or on Justpaste.it if it's too long.
main debug: auto hiding mouse cursor
main debug: auto hiding mouse cursor
main debug: auto hiding mouse cursor
main debug: auto hiding mouse cursor
main debug: auto hiding mouse cursor
qt debug: MRL(s) passed to the Sout: 1
qt debug: MRL(s) passed to the Sout: file:///C:/Users/marco/Desktop/H.265.MP4
qt debug: Transcode MRL: sout=#transcode{vcodec=hevc,vb=800,fps=30,scale=Auto,acodec=none,scodec=none}:std{access=file{no-overwrite},mux=ts,dst='C:/Users/marco/Desktop/NEW.MP4'}
main debug: deleting item `H.265.MP4'
main debug: incoming request - stopping current input
main debug: incoming request - stopping current input
main debug: killing decoder fourcc `hevc'
main debug: removing module "avcodec"
main debug: incoming request - stopping current input
main debug: Creating an input for preparsing 'Converting file:///C:/Users/marco/Desktop/H.265.MP4'
main debug: incoming request - stopping current input
main debug: saving a free vout
main debug: incoming request - stopping current input
main debug: incoming request - stopping current input
main debug: reusing provided vout
main debug: removing module "mp4"
mp4 debug: freeing all memory
main debug: Program doesn't contain anymore ES
main debug: removing module "record"
main debug: removing module "cache_read"
main debug: removing module "filesystem"
main debug: dead input
main debug: nothing to play
main debug: destroying useless vout
main debug: removing module "direct3d11"
direct3d11 debug: Direct3D11 resources destroyed
qt debug: IM: Deleting the input
direct3d11 debug: Direct3D11 device adapter closed
direct3d11 error: SetThumbNailClip failed: 0x800706f4
direct3d11 debug: Win32 Vout EventThread terminating
direct3d11 debug: Win32VoutCloseWindow
direct3d11 debug: WinProc WM_DESTROY
main debug: removing module "inhibit"
qt debug: releasing video...
qt debug: video widget is released
main debug: removing module "freetype"
main debug: removing module "yuvp"
main debug: removing module "swscale"
main debug: looking for meta fetcher module matching "any": 1 candidates
lua debug: Trying Lua scripts in C:\Users\marco\AppData\Roaming\vlc\lua\meta\fetcher
lua debug: Trying Lua scripts in C:\Program Files\VideoLAN\VLC\lua\meta\fetcher
main debug: no meta fetcher modules matched