If you're testing with rechargeable batteries, even freshly charged, try it with brand new alkalines. NiMH cells don't last for ever and they can fail by losing nearly all of their capacity or getting a high internal resistance. Either of those will cause the camera to see a low voltage.

Assuming alkalines from a new pack don't help, at this point you've got nothing to lose by trying a couple of things. Both of these risk damage or even total destruction

Battery Terminals

Even though you've checked, I would want to get into the battery compartment and look not for rust but for the corrosion caused by leaked batteries. Rust (iron oxide) is brown, but what you'd get if batteries had leaked is much paler and harder to spot deep inside a battery compartment. "Cleaning" as you say you've done with a dry or damp cloth, or any likely chemicals, won't do much.

Some people say you can remove it with a pencil eraser but I've never had much luck. very fine sandpaper is the way to go, either on the end of a flat rod or after disassembly. You need to remove every trace from the contact points. It's highly insulating and even a speck will prevent contact with the conductive metal.

External Power Supply

Canon made an external power supply for this camera, part number ACK600. It may still be available, or third party equivalents are; they claim to be 4.3V (surprisingly low given that alkaline batteries add up to 6V and NiMH to 4.8V). It has a fairly standard barrel connector. If you have a selection of old power supplies, as I do, I'd look for one of 4.3V or slightly more (but definitely not over 6V, and don't put batteries in at the same time if >4.3V in case it tries to charge them) and a suitable connector, and splice them together. I can't find the polarity online but it may be marked on the camera. If not I'd try centre positive.