Are you changing SSL settings? The only cause I know of for what you are describing happens when you disable SSL, it can confuse your browser. To help prevent confusion, the webgui shows this message:
If you are sure that is not the cause, go to Settings > Syslog Server and temporarily enable "Mirror syslog to flash". The next time it happens:
1) clear your browser's cache to see if that helps
2) if not, reboot the server and upload your diagnostics (from Tools -> Diagnostics) here
Note: once you past this issue be sure to disable "Mirror syslog to flash" as it adds wear and tear to the flash drive