I am using my one bitcoin/CLN node. I am able to open channels with other Lightning nodes, but not this one (ACINQ).
every time I get a "Rate Limit Reached" error. What does it mean exactly?
It means ACINQ has imposed a rate limit, which you've exceeded. It can happen to other nodes as well, it might not be just you. Rate limiting is a mechanism usually used by big servers, as this one, to prevent abuse either due to encountering too many requests within a specific timeframe, or simply because you don't meet their channel opening policies.
I'd wait for a day or two if I were you. If you do have waited, try contacting them:
hello@acinq.co