A damaged rules database can prevent SpamSieve from launching, cause it to report errors, or prevent the Allowlist and Blocklist from working properly. Rules database damage can be caused by crashes or disk errors or if you are storing SpamSieve‘s files in the cloud and using them with more than one Mac at the same time (which is not supported). To fix this:
Quit both your mail program and SpamSieve.
If you have a backup from before the damage occurred, you can replace the folder:
/Users/<username>/Library/Application Support/SpamSieve/Rules.spamsieve-rules
with a working copy. See the How can I open the Library folder? section for how to find this file.
If you don’t have a backup, you can delete the damaged Rules.spamsieve-rules folder, and SpamSieve will start a new one.