Scylla’s troubleshooting section contains articles which are targeted to pinpoint and answer problems with Scylla. For broader issues and workarounds, consult the Knowledge base. Keep your versions up-to-date. The two latest versions are supported. Also, always install the latest patches for your version.
Also check out the Monitoring lesson on Scylla University, which covers how to troubleshoot different issues when running a Scylla cluster.