Thanks simonspa. I was unable to fix this, even with the help of the sysops at the hosting company. There was no further information in php logs or elsewhere. Just a white screen in the browser with the text shown in my OP. It’s not clear if the message is coming from the Nextcloud software or from the server itself.
Fortunately there was a workaround. The hosting company has a one-click installer for Nextcloud and I was able to use that without any problem.