I would take another look at what jeffbij mentioned about balanced and single ended connections. That might still be the problem. Balanced and single ended modes of operation are not compatible. If you want to mix them, you typically have to do something to make both components balanced, or both single ended. With balanced, you have 2 signal conductors (both are hot +). SE only has one signal and a ground. If you want to mix the 2, something needs to be done with the extra signal conductor because RCA/SE can only handle 2 conductors. It's a very common practice to take the extra signal and tie it to the ground pin. Its usually done in an adaptor or cable that functions like an adaptor. When you take something like that and plug it into a component that outputs a balanced signal, you can create a short circuit because you are crossing a +and - somewhere in the connection. This may or may not be a problem. There are different ways a balanced connection can be wired. So, even if you didn't have an issue in the past mixing components, it won't always be the case. You just got lucky.
You need to go over the manuals for both pieces of equipment. Any special requirements you need to make for compatibility will be in the manual. I would do that first because it is possible to damage your equipment without taking the proper steps. Usually, no damage is done if you catch it right away. If you left the components on for a few days without realizing it, damage may have been done. Its probably OK, but I would make sure before I put it back in a system.