Dear JIAZ, I have the same problem in my JDOWNLOADER 2 from about 2 day.

My conditions is the same from about 15 years and JDOWNLOADER 2 was working very well:
PC with O.S. WindowsXP - SP3 - 32 bit & Jdownloader 2 portable and automatically upgraded to v. 23 Jul 2018.

If I download any video from YouTube, the dowload starting, but after download the file an error stop the process with line message:
"Saltato -FFMpeg Mancante"
or "Componente aggiuntivo obsoleto"
(my language interface is in italian)

On seek of the solution I have tryed your idea on the Jdownloader 2 Forum, and I tryed:

1.) Close JDownloader
2.) Delete Core.jar file and the folders tmp and update
3.) start JDownloader and it will update itself to latest available version and restore missing files
after that the avcodec-58.dll file should be available again

Yes, the avcodec-58.dll is again in the directory c:\Programmi\_Jdownloader\tools\Windows\ffmpeg\i386\ (in my configuration).

Now I try for new download of the same file:
- the download not start and the identical error message appear instantly
"Saltato -FFMpeg Mancante" !!

Now I tryed this:

On Jdownloader 2 tab OPTION > OPTION ADVANCED

I search the word FFMPEG and I set:
- FFmpegSetup: Binary Path Probe to NULL
- FFmpegSetup: Binary Path to NULL

I close Jdownloader and and restart it.

Now I start the download again from YouTube, same video.
Jdownloader start the download regularly, BUT at the end of download, the error message appear again:
"Saltato -FFMpeg Mancante" !!

For what this?

This problem is present from 2 day only, and I not changed any configuration of the WindowsXP.

Is possible help to me?

Many thanks for your interest to my problem.

Bye for now.