I have tried downloading the newer version ( [kmymoney-5.1-481-linux-gcc-x86_64.AppImage].
This also exhibits the parsing problem but with a different error message :- “File was not parsable! /home/appimage/Craft/BinaryFactory/downloads/git/extragear/kmymoney/kmymoney/plugins/xml/mymoneystoragexml.cpp:1411”
The differences in the error just reflect that they were compiled in different environments, and against slightly different versions of the source. That should not matter.
Where are the files you are workind with? Local hard drive or network share? Permission issues don’t seem likely, but should be ruled out. What type of files are you using? xml, kmy? What does “ls -l” show for the working and not working files? What does the “file” command say about them?