WHMCS Errors Diagnosis and Fix

My WHMCS websites are encountering several issues unexpectedly. The add funds module is non-functional, there are transaction view problems in the client area, and login attempts from various devices are failing. I am unsure if there are specific error messages associated with these problems.

I need a professional with WHMCS experience who can diagnose these issues and implement the necessary fixes. Timely communication and progress updates are crucial, as these problems are impacting my clients' experience. Thanks!