Reinstalling apache and its modules shouldn't be problem. Have you doublechecked that there aren't any outdated modules installed/active?
You should use proxy_fcgi and proxy from the same Debian repository / version as apache.
I've removed the package via
aptitude purge libapache2-mod-proxy-psa
I didn't get any dependency warnings but keep this in mind and make sure that no other (used) packages will be removed.
Had the same isssue with the following setup:
Plesk Onyx Version 17.8.8
Debian 9.3
Apache/2.4.25
These steps solved the problem on my system:
Purging an outdated libapache2-mod-proxy-psa (initially installed while using Debian 8.x)
Disabling the Apache modules proxy_fcgi and proxy...