The normal Cause of this is you have a poor connection somewhere between your panels and the MPPT, like a loose, corroded or oxidised MC4 connector causing a high resistance joint. Sometimes it is a failing panel with a failing joint inside, but usually the wiring.
When you disconnect the panels from the MPPT they are open circuit so even with a high resistance joint the voltage gets up to 60V. A voltmeter draws very little current. When you connect the MPPT, it tries to draw full current from the panels so you get a lot of voltage drop over the poor connection and the input voltage drops to just over the battery voltage.
An open circuit voltage test is not good enough to test if a set of panels is working due to the effect above, you need to do a short circuit current test as that would show a much lower current than the spec due to the high resistance. However, not many people have the kit for this if Isc is above 10A which most basic multimeters are capable of.
Go over your system bit by bit.