1. The phone paused the page
What you see: the progress stops, or the job fails with a message about the page going into the background.
Phones save battery by pausing pages you aren’t looking at. Switch to another app, take a call or let the screen lock, and the browser stops the video decoder and encoder that were doing the work. iPhones do this most firmly. FitVideo keeps the screen awake during a job and, when merging videos, carries on from the last frame when you come back. But a stopped encoder can’t be resumed, and the job has to start again.
Fix: keep the page open, with the screen on, until it finishes. For long videos, plug the phone in. A computer avoids the problem entirely.
2. The file is still in the cloud
What you see: “Your browser lost access to the video file”, often part-way through.
When you pick a video from Google Photos, Google Drive, OneDrive, iCloud or a chat app, the phone may hand over a file that is still being downloaded in the background. If the download pauses, the browser can no longer read it. FitVideo copies chosen videos into memory straight away, up to a size that depends on how much memory the device has, and tries once more by itself if a file stops responding. Very large cloud files can still fail.
Fix: save the video to your device first (in Google Photos, Download; on a computer, right-click a OneDrive or Drive file and choose Always keep on this device), then choose it again.
3. A special iPhone format
What you see: “Your browser couldn’t read this video’s picture.”
Recent iPhones can record HDR (Dolby Vision), Cinematic mode and spatial video. These are still HEVC files, but with extra layers that not every browser’s decoder accepts, especially on Windows and older Android phones. Ordinary iPhone HEVC usually works; see iPhone videos not playing on Windows for that.
Fix: try Chrome or Edge on a computer. To avoid it in future, set the iPhone to Settings, Camera, Formats, Most Compatible, and turn off HDR Video under Settings, Camera, Record Video.
4. The browser can’t read the sound
What you see: “Your browser can’t read this video’s sound”, usually when extracting audio or converting to MP3 on an older iPhone.
Safari before iOS 26 has no WebCodecs audio decoder. FitVideo works around it by copying the sound out first and decoding it another way, which works for most videos but not all.
Fix: choose M4A instead of MP3. An iPhone video’s sound is already AAC, so M4A copies it as it is without decoding anything. Updating iOS also fixes it. More in extracting audio on a phone.
5. The computer’s graphics chip refused the frames
What you see: on Windows, a job that fails with “Encoding error”, or that suddenly runs slower than expected.
Browsers use the graphics chip’s hardware encoder to make video quickly. Some graphics drivers reject frames that were drawn by the page, which every resize, watermark, merge and photo video does. FitVideo spots this and makes the video again with the browser’s software encoder, then stays in software for the rest of the visit. It’s slower, but it works.
Fix: usually nothing; the retry is automatic. If it still fails, update the graphics driver from the PC maker’s or the chip maker’s site.
6. It isn’t the kind of file the tool expects
What you see: “That’s a sound file, not a video”, “That’s a photo, not a video”, or a format that can’t be read.
This happens more often than you’d think: a voice note given to a video compressor, a screenshot given to a converter, or a file from a camera or app that uses a container browsers don’t read (AVI, 3GP, some security-camera formats). FitVideo reads MP4, MOV, WebM, MKV and M4V.
Fix: for sound files, use the audio converter or audio cutter; for photos, photos to video. For other containers, a desktop app such as HandBrake (free) converts them to MP4 first.
7. The video engine didn’t load
What you see: “The video engine didn’t load.”
The part of FitVideo that reads and writes video files is a script the page downloads. A dropped connection, a strict ad or script blocker, or a company network can stop it. FitVideo tries its own copy twice, then an identical, integrity-checked copy from a public library server.
Fix: check the connection and reload. If you use a script blocker, allow fitvideo.app and unpkg.com, where the backup copy comes from.
8. The browser is too old
What you see: “Your browser can’t encode video yet”, or a browser that can’t make a video at the size you chose.
In-browser video tools need WebCodecs, a fairly recent browser feature. Old tablets and phones that no longer get updates often have a browser that can play video but not make it, or can only make it at small sizes.
Fix: update the browser, or use a newer device. For greetings on an old device, saving as a GIF often still works.
9. The result is too big for the browser
What you see: the job stops near the end of a very long or very large video, or the device runs out of memory.
A browser keeps the new video in memory until you save it, and a single block of memory can’t be bigger than about 4 GB. Phones run out well before that.
Fix: make the result smaller. Compress to a size instead of converting at full quality, trim the video into parts, or close other tabs and apps first.
Still stuck?
Use Report a problem on the error message. It sends the error code and your browser version, never the video, and we read every one. Several fixes on this page started as a single report.
Free toolTry it now: Video compressor Any size, any videoQuestions
Why does my video stop processing when I switch apps?
Phones pause pages in the background, which stops the video work. Keep the page open with the screen on until it finishes.
Why does a video from Google Photos fail?
It may still be downloading from the cloud when the tool reads it. Download it to your phone first, then choose it again.
Is my video uploaded when a job fails?
No. Only the error code and browser version are recorded, so we can fix the cause. The video stays on your device.
Which browser works best?
Chrome or Edge on a computer. Recent Safari and Chrome on phones work too, as long as the page stays open.
What does “The encoder stopped partway” mean?
The browser’s video encoder stopped before the end, often because the page went to the background or the device ran short of memory. Try again with the page open, or with a smaller target size.