How to fix MuseScore 4 corrupted files

MuseScore 4.0.0 had a bug that caused files with part scores to become corrupted when inserting or removing measures or changing the time signature. Often, in affected scores, some measures (especially with local time signatures) have an incorrect actual duration.

MuseScore 4.0.0 and 4.0.1 do not yet show details about where the corruptions are, like MuseScore 2 and 3 did. This is fixed in MuseScore 4.0.2 and later, along with quite a few bugs that could cause corruptions.

Scores from MuseScore 4.2 and later don't open at all in 4.1, and entirely empty in 4.0

Here are some methods to fix corruptions:

• If MuseScore 4 can open the file, install and enable the plugin Repair Assistant, then open the score, ignore the corruptions and run that plugin. With some luck this already fixes it.
• If MuseScore 4 can open the file, but the above step didn't (fully) help, then export it as MusicXML (or Save as an uncompressed folder (experimental)). Open the MusicXML (or the mscx) with MuseScore 3.6.2 (but see below, when using MuseScore 4.2) to find and correct the corrupted measures. Save, open with MuseScore 4. It doesn't necessarily work though.
• If MuseScore 4 cannot open it, use WinZIP or 7Zip or something alike to open the mscz file (which really is a ZIP archive in disguise), extract the mscx and open that with MuseScore 4 (or it that can't open it, use 3.6.2 instead). You can also extract the score_style.mss and use Format > Load Style on it. If using M3, you need to ignore the warning reg. them being from a newer version of MuseScore and would need to reset the style of title, subtitle, lyricist and composer. Note: as of MuseScore 4.2 that won't work anymore, you'd either need to to delete the <LastEID>...</LastEID> line from the mscx or use MuseScore 3.7 Evolution, which reads the mscz file directly, it also reads and converts all styles correctly.
  You may want a Ctrl+A,Ctrl+R after that to reset all elements' position to their defaults.
• If there are corruptions in parts (like MuseScore 4.0.2 or later reports them), it may help to go to File > Parts and in the parts' ... menu select the "Reset" option, save (ignore the corruption message), close, reopen.

If you're still getting reports about corruptions, continue with How to fix a score that contains corruptions.