Hall of Famer

Hi there, recently I've encountered an extremely strange problem with my WHMCS website(mysidiahost.com). It used to work properly, but now it just behaves absolutely strange. First of all, registration for guests wont happen. Guests can fill out the form and click submit/register button, but their records will not be added to database, instead they are being redirected to login page. Creating new clients in Admin control panel wont work either, the client wont be created. Also, existing clients cannot log into client area. They always get password incorrect error, even if their passwords are entered correctly or just been reset. I can still log into Admin control panel though, this does work properly. Its extremely strange since a few days ago my WHMCS site was still working, and then this happened out of nowhere so I am totally clueless. So do you know under what circumstances that client area/registration can fail completely like this? The only changes happened to my server recently was that it was upgraded and migrated to a new server. I recently read that mod security settings can lead to this problem, so I am trying to confirm this with my dedicated server hosting provider. But anyway, did anyone experience the same problem before? If so, how did you fix? Also I opened a ticket for this one but no response in 24 hours. Why does this happen?

This is the screenshot, as you see it really does not work.

Well I have setup WHMCS through softaculous, everything worked fine except domain registration. As you can see from the provided screenshot, the dropdownlist for suffix has nothing. It should at least contain something like .com, .net, .org, etc. I found out this problem when I was trying to add my subdomain to the dropdownlist, but now it has absolutely nothing. Is this the intended behavior? Or is this a glitch? How can I resolve the issue? Can anyone please help me? oi63.tinypic.com/34t9a82.jpg