Motion artifacts after re-encoding

[PlayingTarot]

Hello, everyone.
I'm converting videos saved from nicovideo.jp (hardsubbing and re-uploading them on Youtube).
I have a problem with my last video. There's quite a lot of motion and explosions in the video, but there is only one place in the video where I get artifacts (glitches?).
Let me show you the screenshots (KMPlayer). This is the source image

Almost instantly, the camera moves to another angle:

That's the point where I get horrible blocks, look at this (converted video):


I don't know whether it is related to the motion of the picture or something else...
For conversion, I use Avidemux Mpeg4 AVC (x264), Constant Quantiser Single Pass (set to 0, for lossless), in mp4 muxer.
It should be noted that the source video preview in Avidemux window also displays this scene with glitches.
I did almost everything for increasing the conversion quality (the output size doesn't matter much for me).
I don't know how to get rid of these artifacts, please help!

[poisondeathray]

If preview in avidemux shows problems, then avidemux isn't decoding the video properly . Use a different program to hardsub that decodes it properly

[jagabo]

AviDemux isn't decoding your source properly.

[PlayingTarot]

That's very bad news, because I've been using Avidemux for quite a while and it worked great until now.
And the other part of the encoded video looks flawless, almost as the source.
Is there anything that can be done with Avidemux? Other codecs or settings, perhaps...
If not, what program would you recommend to replace Avidemux?

[poisondeathray]

You can't change the decoder in avidemux. You can try another version, or maybe they'll fix it eventually

Popular alternatives to hardsub with a GUI are megui, xvid4psp, handbrake

[PlayingTarot]

Well, I've just tried xvid4psp, and that explosion scene looks alright now, but now I get this mess in the beginning of the video (which looked alright in Avidemux). Again, both in video preview and the converted video. In addition, the audio track gets sped up greatly (I experienced it with some converters in the past, but can't remember what it was caused by...).
Handbrake apparently does not support ASS subtitles, which is crucial for me...

[poisondeathray]

You can try changing the decoder or script used in xvid4psp. I think it's avisynth => edit filtering script and you can preview it. Copy & paste it back here

You don't need to waste time encoding. If the preview is already messed up, the encode will be messed up too

Or use something else like megui

[PlayingTarot]

Well, thanks! Changing the decoder seems to solve everything, though I changed it through Video=>Decoding=>Other files, from DirectShow Source to ffmpeg Source. Here's the filtering script after the decoder change btw.

import("C:\Program Files (x86)\Winnydows\XviD4PSP5\dlls\AviSynth\functions\AudioFunctions.avs")
import("C:\Program Files (x86)\Winnydows\XviD4PSP5\dlls\AviSynth\functions\VideoFunctions.avs")
loadplugin("C:\Program Files (x86)\Winnydows\XviD4PSP5\dlls\AviSynth\plugins\bass\bassAudio.dll")
loadplugin("C:\Program Files (x86)\Winnydows\XviD4PSP5\dlls\AviSynth\plugins\VSFilter.dll")
loadplugin("C:\Program Files (x86)\Winnydows\XviD4PSP5\dlls\AviSynth\plugins\FFMpegSource.dll")

FFmpegSource("C:\Users\bearm_000\Desktop\mmd\reimuaya\reimuaya.mp4", atrack = 1)

AutoYV12()
TextSub("C:\Users\bearm_000\Desktop\mmd\reimuaya\reimuaya.ass")

The output quality seems alright, too. I think about using xvid4psp in future. Though I had to use an older version (5.037), because I got lost in the interface of the latest version...
Again, thanks for help, everyone!