I don't know what I did to arrive at this impasse, but all of a sudden I cannot open my file. (Using: kmymoney-5.1-492-linux-gcc-x86_64_494030f0808318425d6a9b17bd6457f4.AppImage in Fedora)
I get the following error:

File was not parsable! /home/appimage/Craft/BinaryFactory/downloads/git/extragear/kmymoney/kmymoney/plugins/xml/mymoneystoragexml.cpp:1411

I tried (a) going back to a previous version of the program, (b) renaming the data file to *.xml.gz, unzipping it, and trying to load that expanded file, (c) tried a backed up version of the data from an external drive, and of course the mandatory (d) rebooting. No joy.

Any ideas? thanks, A

A little more investigating shows that it is the kmy-file plus the 4 latest ones behind it that are corrupted. I was able to go to *.kmy.5~ and get back up and running. If anyone knows how I could have committed this faux pas, I would appreciate the knowledge.
Thanks,
A

Since you know how to convert a gzipped .kmy file into the corresponding .xml format, you can do that for the last one working and the first one failing. Then you can use the diff command line tool to see the differences. Maybe, that shows the problem. You could also run xmllint on both to see if that shows any errors and report back.

Hint: zcat xxx.kmy > xxx.xml is a quick way to get to the xml file format.

Thanks, IPW.
I tried diff and also some of the sexier gui versions but there are so many differences that it is impossible to wade through. I piped diff jpgood8.xml jpbad6.xml > diffs and that diffs file was as large as the sum of the two original files. (40 megs. !)

On the other hand, your suggestion of xmllint solved the problem. When I run xmllint on the bad file I get:
________________________________
~/Documents/KMyMoney> xmllint jpbad6.xml
jpbad6.xml:167261: parser error : Extra content at the end of the document
<PRICEPAIR from="E000059" to="USD">
_________________________________
I investigated that line (167261) and there were many many lines after that.
It turns out that there was an end-of-file marker (</KMYMONEY-FILE>) that was misplaced -- roughly half way up in the file. I moved it down to the eof, and the file loads. I don't know how that could have happened.

In any case, just to be on the safe side, I have gone back to the most recent "good" file and updated it.

and thanks for the tip about zcat, that is slightly faster than gunzip...
A