You should add some info: Which server software do you use? What version number? And maybe how you installed this, Docker?
For example “Synapse v1.119.0” That’d be the most recent one.
~~If I’m not mistaken Synapse needs an additional proxy server for sliding sync? https://github.com/matrix-org/sliding-sync/blob/main/docs/Landing.md ~~
I believe they’re still messing with sliding sync and switching that over to a new version of the protocol. There are some open issues with Dendrite:
https://github.com/element-hq/dendrite/issues?q=is%3Aissue+is%3Aopen+sliding
https://github.com/element-hq/dendrite/issues/3236