A common error is that sometimes when certain pages display an internal server error and there is no stacktrace. If there was a custom theme before the upgrade the page layouts will still reference them and if one has not installed or recompiled the layouts in a new sdk it will create a internal server error.
The Solution is to recompile the layouts / themes in the new plugins SDK. Another quick workaround until then, is to switch to a default page layout by doing the following:
- Go to the page or view site pages from control panel.
- Select Manage Pages.
- Change Layout to a default Liferay layout.
An easy test is to change layouts, as this usually only occurs just after an upgrade.