I hope that this helps someone.
I had recently re-installed my studio computer on Ubuntu 20.04, with the Low-Latency Kernel.
I have used Rosegarden for years, and tried to open an existing file.
And immediately Rosegarden would crash.
I could open Rosegarden and create a short four part MIDI project.
Save it and close it. Then try to open the same file and Crash.
I checked the Logs app and found messages where I could not find any reference.
After days of research I found nothing. No solution. Only reference to the same issue in 2010.
And that Rosegarden should be upgraded to a newer version that is way behind the current version.
Today I think I have found the solution through many trials and Crashes.
Open the Terminal: Ctrl+Alt+T
Then run Rosegarden using: sudo rosegarden
Rosegarden opens. Then allows me to open any existing file.
Edit a file and save. With no more crashes.
My assumption is that using the sudo command I am giving Rosegarden more privileges.
Which seems to over come the issue it has when running from the icon.
I have been able to repeat and reproduce this success over and over.
I hope this helps someone.
UPDATE
If I right click on each .rg file and check Permissions.
Owner is Me with Read and Write permissions.
And the Group is my user name.
All of the .rg files in my Home / Music show the same.
If I double click on a .rg file. The Rosegarden application opens, then crashes.