You can incur a number of issues with Java related to the P6 Web application. Here are a few quick things you can do to troubleshoot.
1. Clear the browser history and cache.
2. Clear the Java Console files. Control Panel>Java>General>Settings>Delete Files.
3. Check the version's tested configurations to make sure you have the right browser, jre, and OS. Usually the culprit may be that you are using an incompatible version of the jre.
4. If using Firefox, make sure you are using a 32-bit jre as some versions of Firefox are 32-bit.
5. Make sure that when the jre is correct and the browser is launched that the browser is enabling the jre. On Firefox, you can check add-ons. On IE, you can check the control panel>advanced to make sure IE is picking up the jre.
6. Increase JVM memory for better performance.
7. Run time Java memory
8. Disable certificate revocation checks
9. Users must have unique Java cache area etc.