Three test:

Are you banned from that community?
easy way to check: just try to access it while logged out. If you can, then you’ve been banned.

Did either instance block the other?
Nope.

How to check:
lemmy.sdf.org/instances
lemmy.zip/instances

Are you accessing Lemmy through a phone app instead of a browser, and is that app blocking that community or instance?
Easy way to check, just use a browser.