I know this answer will not seem too much of a solution. Initially it came to what is said here ("It happens every now and then. You have to wait "
): after more than one restart, and a few hours, it worked.
(I realized that even adding the first account was problematic in the same sense: it didn't work the first time and the blank window was displayed; but then it did work a few minutes after, at a second try. Now it didn't work after a second, third or fourth try, but did after a few hours.)
I don't know what to make of this.
And after a while, the first added account stopped working completely, no upload possible for the opened document and then no access to online files, while the second account worked. Removing both and adding the first, it had no access to the file I was working on when the error occurred, while other files were accessible. Accessing that document with a browser and downloading it, I see it is ok, excepting that (as it was to be expected) it hasn't the changes made after the last successful upload. But there is no way I can open that online file from Dolphin.
I think this tool is broken in 20.04, and unsafe for live editing anyway.