Invalid file after applying a transition filter

[insam]

Hello, I apologise for having to use a translator and for the fact that I'm not very good at it.
Every time I apply a fade to black at the end of a video before pasting it to another, the pasting is no longer possible, well it's done without any errors being reported, but the resulting video is invalid from the pasting onwards.
How do I do this?

[eumagga0x2a]

You probably try to combine incompatible videos in copy mode. To append videos in copy mode, not only basic properties like width / height and video codec but also the entire decoder configuration AKA codec extradata must match perfectly. You will have to re-encode.

Usually, Avidemux warns when source videos across cut points don't match. Have you enabled the silent mode thus suppressing useful warnings?

[insam]

Thank you for your reply
No, I didn't even know there was such a mode.

I've sometimes had this warning, when I've actually tried to concatenate files from different origins. But for now I'm just pasting short episodes of a series that were all recorded in the same way, and which I can paste perfectly well as long as I don't add a filter between them. When I add a transition filter, I assume I'm changing something, but what? How do I know what to change?
Thanks in advance if you can help me, I've been using Avidemux for so many years because it's the only one whose interface I more or less understand, I've never encountered this problem (but I rarely use the filters either).

[sark]

Every time I apply a fade to black at the end of a video before pasting it to another, the pasting is no longer possible,

To clarify, are you saving the fade to black video, before re-importing to append to another video. If so, you will have videos with different parameters. As eumagga0x2a said, this will only work with additional re-encoding. Not in Copy mode.

[insam]

Yes, I'm obliged to save it since it's not possible to insert the transition between the first file and the added one.

But thanks, I understand what you're saying. I've found a bastardized solution that allows me to avoid wasting time re-encoding all the videos: I only apply the transition I need to an extract of the video, I play the modified extract by filming it with the same parameters as the other files, and all I have to do is process this new video in Avidemux by cutting the excess at the appropriate cut points.

At least I've learned something, thanks.