Hi,
I tried it, because the only thing I could loose is time.
And it works !
My problem was that my old server is/was ubuntu 16.04. There the standard php is 5.6. So I installed 7.3 and enabled fcgi to activate it for owncloud. There is works.
On ubuntu 20.04 the standard php is 7.4, and owncloud is not ready for that, so I activated 7.3 in the same way as on ubuntu 16.04.
Now I have php7.3 as standard. I have to check if other virtual servers still work.
So, thanks for the idea to try.