I should mention I've already tried
yum reinstall cwp-suphp
I've also rebuilt the vhosts config, and I have tried to rebuild apache (though the 2.4.29 version that installed during the upgrade is not available in CWP; 2.4.26 is the latest version in the Apache Builder).
Also, there are no errors on httpd start. Browsing to php pages generates a 500 error.