From my research the following is an issue that has persisted for at least three years.

On PS5, I tried to download my world from my realm, however upon reaching the end of the download I am met with an 'Error, An error occurred' message. Unlike some other threads have commented, my download does not get stuck at 650 MB. It just continues up to 770 MB (the size of the world) and displays the error message.

There seems to be no fix.