Edit added:
I was able to fix the problem although I'm not exactly sure how.
I opened the .kmy file on a new instance of KMyMoney on another machine and updated it with new data. I saved the new file.
I went back to the problematic KMyMoney instance and was able to use this updated and different .kmy file to successfully view my data.
Maybe the file itself was the problem.
This issue is resolved at least to my satisfaction.

KMyMoney version 5.0.8
On Linux Mint xfce 20.3

First starting KMyMoney in the Welcome to KMyMoney window.
I select "Open an existing data file" and select to open my backed up .kmy file from the directory where it's stored.
A green bar fills out at the bottom of the Welcome to KMyMoney window as though it is opening the file just fine.
But my data does not appear on the Home page as it should. Instead, the Welcome to KMyMoney display persists.

run "kmymoney"
When the gui comes up I select "Open an Existing Data File" and select the .kmy file from a directory.

last lines of output on the terminal are:

start parsing file
startDocument
reading securities
endDocument
Cost center model created with items 0
Payees model created with items 0
qt5ct: D-Bus system tray: yes
kf5.kdbusaddons: Can not find 'kdeinit5' executable at "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin" "/usr/bin, /usr/lib/qt5/bin"


run "kmymoney -n"
When the gui came up, I chose Open An Existing Data File and selected my .kmy file.
Now the last lines of output from the terminal are

kf5.kdbusaddons: Can not find 'kdeinit5' executable at "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin" "/usr/bin, /usr/lib/qt5/bin"
reading file
start parsing file
startDocument
reading securities
endDocument
Cost center model created with items 0
Payees model created with items 0
qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 1644, resource id: 8979064, major code: 40 (TranslateCoords), minor code: 0

Thanks in advance with any help so I can see the normal Home page with my data!