I am using the same live connect method to communicate between the applet and Javascript. I came across many different warning screens throughout the past few years from JRE6u45 until the current JRE8u131. It depends on which warning screen you are now facing.
Did you use Firefox every time? Firefox does not support the Java plugin since a certain version see here. FYI, Chrome and MS Edge do not support Java Applet. Please check if you are not using them.
The Caller-Allowable-Codebase attribute should not trigger any warning. It should block the applet from running if it does not include the domain correctly. There should be no warning message that allow you to run.
Generally most warning screens come from the JRE instead of the browser. So you may also check the following items:
- the code signing cert is valid and not expired.
- Go to Java Control Panel, clear the applet cache (In General Tab)
- Enable Java content is checked (Security Tab)
- Restore Security Prompts (Security Tab)
- Enabled mixed code to show warning if needed (Advanced Tab)
- Enable the logging and tracing, show Java console (Java Control Panel -> Security Tab)
Please provide more details screen or warning messages you have.