I don't use the horde portal much at all any more, but I get an error as well, when clicking on add content. On my test server, I fixed it by editing line 167, and changing &new to new. I changed it on my production server as well. Now add content works.
If you don't get anything when running pear list -c horde, then something went wrong. The packages that I update, are not compatible with any of the horde5 rpms from remi, which it seems you have previously installed on the production server. I believe his changes the pear directory location, and adds a lot more information to the pear.conf file that I don't know about. Somehow, you are going to need to resolve those conflicts. How, I'm not 100% sure what the best way is. None of my horde5 rpm's start with php-horde, so that is how I know you have installed someone else's packages.
I have recently started using the software collections contrib, and am running my production server with php73 enabled. I did have to make some small changes to reduce some of the log errors, but I didn't get rid of them all, as well as copy the current php scl pear.conf files, and create a symlink to the one I use for webmail. I'm letting all of this soak on my home system, but all seems good so far. I'll have to retest the portal fix on a base install, and then with the various php SCL versions enabled. All of these changes will be included the next time I upload the files, so more people can test and confirm things are good.
I don't know how much, or if any, that I've helped here.
John