As a security measure, activity on patient or client accounts is monitored, and excessive activity results in a temporary shutdown of access to the account. This is to prevent "hackers" from doing mischief in accounts, but as you see, sometimes it also prevents legitimate users from having access.
You will notice that, if you try to access the client accounts from the same computer that you use to access therapist accounts, the excessive activity warning will appear. Since you access the system a great deal, your own computer is already over the limit for client access. This does not mean that they will be shut out on their own computers at home.
If the problem you are experiencing is related to this "excessive activity" issue, be aware that the system resets the activity counter after 48 hours; no action is required other than waiting.
Clients may experience a different issue if they try to set up an account for themselves when an account already exists. In order to prevent duplicate accounts from being created, the system looks at clients already in the system, and prevents the creation of a new account with the same last name and birth date. This is not a bad thing: having duplicate accounts can create all sorts of confusion. If you want a client to register themselves, don't set up an account for them ahead of time; let them create the account from scratch themselves.