Avidemux Save Errors Explained: “Too Short”, “Muxer Cannot Open”, “Unsupported”

Avidemux save errors almost always mean the same thing: the combination of your video codec, audio codec, and output container is invalid. The error dialogs use terse, developer-written wording — “Too short”, “Muxer: Cannot open”, “Unsupported” — that tells you what failed but not why. This page is an error-message encyclopedia: find the exact message you saw, learn the cause, and apply the fix. Each section below uses the verbatim message text so you can match it to what is on your screen.

Error 1: “The video has been saved, but seems to be incomplete (Too short)”

What it means: Avidemux wrote the file, but the timestamps in the video stream are broken — missing or colliding decode timestamps — so the file plays shorter than expected or players choke on it.

Common causes:

• Cut points placed on non-keyframes in Copy mode, producing timestamp collisions at the boundaries.
• A corrupt or unusual source file (common with phone recordings and some screen captures).
• Certain codec/container pairings — forum reports show HEVC video into the MP4 muxer triggering this where the MKV muxer worked fine.

Fixes to try, in order:

1. Move your A/B cut points to keyframes (use the ↑/↓ keys) and save again.
2. Switch Output Format to MKV Muxer — it tolerates timestamp quirks the MP4 muxer rejects.
3. If it still fails, re-encode (Video Output to x264) instead of Copy: re-encoding regenerates clean timestamps.
4. Suspect a corrupt file? Run it through ffmpeg -i input.mp4 -c copy fixed.mp4 first, then open the fixed copy in Avidemux.

Error 2: “Muxer: Cannot open”

What it means: the output container (muxer) refuses the streams you are trying to put into it. The classic trigger, documented on the Avidemux forum: opening a WMV file with Video Output = Copy and Output Format = Mp4 Muxer — the MP4 container simply does not accept WMV video, so the muxer cannot open.

Fix: you must re-encode. Set Video Output to an encoder Avidemux supports for that container (for MP4: Mpeg4 AVC (x264), Mpeg4 ASP (xvid4), or HEVC where available), set Audio Output to AAC, and save again. “Cannot open” is never fixed by retrying the same settings — the combination itself is invalid.

Error 3: “Unsupported — Only MP4Video, H264, and H265 supported for video”

What it means: same family as Error 2, but the dialog names the allowed codecs explicitly. Your source video codec (e.g. WMV, VP9, AV1 in some builds) cannot be placed into the chosen container in Copy mode.

Fix: identical — re-encode to a supported codec instead of Copy. Check File > Information (or Properties) to see your source codec before choosing the output settings, so you are not guessing.

Error 4: filters or Save greyed out

Not a dialog, but the same root cause family: with Video Output = Copy, all video filters are disabled by design (filters need to re-encode), and Save can appear unavailable after certain filter operations. The fix is one click — select a real encoder under Video Output. We have a dedicated page for this: Avidemux filters greyed out? Why it happens and how to fix it.

General triage checklist (any save error)

1. Read the exact message. Match it to the sections above — the wording tells you which combination is invalid.
2. Check File > Information. Know your source video/audio codecs before choosing output settings.
3. Try the MKV muxer. It is the most tolerant container Avidemux offers; if MP4 fails, MKV often succeeds.
4. Re-encode as the fallback. Copy mode preserves the source bitstream verbatim — including its problems. Re-encoding regenerates clean streams.
5. Rule out a corrupt source. If the same error hits every output format, the input file itself is likely damaged; repair it with ffmpeg or VLC first.

Related: cannot find a demuxer (open-time errors), Avidemux not responding / crashes.