Unable to parse class file - it is error, in log it is specified so (but when I put it here text “error” was removed, due to xwiki syntax), but as I mentioned probably it should be warning according behaviour.
I mentioned just this one, because there were plenty of these ones, but it seems this one is not issue.
But there are no other significant errors in logs, so I have no idea what is going wrong when applet is loading and suddenly it “crash” - and then there is just white page.
In older versions there are of course other errors which I just briefly mentioned. But I did not dig into it deeper, I believe you would tell me that Xwiki 8.3 is very old, so I did not put much effort into it.
Concerning my exmaple of differences (Tomcat vs Weblogic), yes it was container specific, see discussion1 and discussion2