If you have changed the templates, please do check them. This error message may show up because of that. If you have not changed them, and you are either installing JSPWiki for the first time or have changed configuration, then you might want to check your configuration files. If you are absolutely sure that JSPWiki was running quite okay or you can't figure out what is going on, then by all means, come over to jspwiki.org and tell us. There is more information in the log file (like the full stack trace, which you should add to any error report).
And don't worry - it's just a computer program. Nothing really serious is probably going on: at worst you can lose a few nights sleep. It's not like it's the end of the world.