I had some issues with WHMCS at the start, so I rolled back to php 7.4 at that time, and still run it under 7.4. There are also some issues with the IP that WHMCS sends when it calls home, and this also affects some other plugins.
Make sure you go over your ini file with a fine-tooth comb and check for proper syntax. Start with everything inside the web directory, then move out all the folders and files that WHMCS recommends in their security settings.
I also only run WHMCS on Apache.