My computer is the only one able to access a clients account

[marleehagerstown]

Hello there,

I'm having an issue that I'm trying to help a client solve but I can't quite figure out what the fix might be. 

I have access to the admin side of WHMCS and our business utilizes the online payment system thru WHMCS as well. (accounts.BUSINESSNAMEHERE.com)

Our clients log-in credentials is the e-mail address they've registered with us as well as the password that they chose. 

Currently I'm working with a client who is unable to log-in to his account. I've attempted to reset his password twice- once with one he picked out and once with one I picked out. For some reason I'm able to log into his account from this computer just fine. But when he, and anyone else try to log in from a different computer or phone, we're unable to.  I'm curious if anyone knows what might be causing this and what the fix may be?

I appreciate the help in advance, thanks!