Hi I would try the following:
1) Export your whitelists to a .bwl file. Save it somewhere other than CAS directories
2) Uninstall CAS saying yes when asked whether you want to delete all settings and whitelists
3) REboot
4) Check all directories (under Docs and settings as well as program files have gone)
5) Backup registry and do a registry clean - Comodos tool works pretty well
6) Reboot again & re-install CAS
7) Inport settings automatically, answering 'yes' if CAS asks if it can correct any account settings.

Check CAS whitelist for any duplicate email addresses, & any differing only by domain suffix (eg .net, .com). If you find them export to .bwl, edit and re-import. This iknown bug s a real risk bearing in mind domains you are changing from and to, and the check must be done AFTER reinstall, as install adds to whitelists.
If all this fails then there is something deeper going on, and I'd file a bug report with as mucch detail as possible - see guidelines - previous and versions of client and CAS, text of bug messages, any new email plugins etc - so everyone can help.
Best wishes
Mouse