When in my results I get PR: n/a, L:eror, LD:eror... does this means that this sites have no PR and backlinks or that pulugin can't find out? Hope I was clear
check on yahoo site explorer for no. backlinks, and there are a number of firefox plugins that can tell you PR and alexa. PR of n/a just means that there havent been any PR updates since the site was created (that or you've been banned by google).
if you are seeing n/a in the PageRank column your site or the page you are looking at does not have a PR yet, this is typically due to your site being new or not yet being fully indexed by google, yahoo, bing, etc..
Whenever I get the error message I assume it's because I've made too many requests and Google is starting to get ticked off and is about to block my IP. Again, this is just an assumption, but it's based on the fact that I generally have this happen after I have been using the plugin a bit too long.