There is an error occurring, when I try to refresh the list of remote gems. The netbeans wiki points out, that this is probably due to an error in rubys gem tool itself and proposes disabling the fetching of details as a workaround. This works but is somewhat dissatisfying. 

Now someone on the german ruby forum pointed out, that the error only occurs if you fetch a list of all the remote gems including details. Narrowing the query to, for example only gems beginning with "cuc" will give me a list of cucumber-related gems including the description. The "Fetch Detailed Gem Description" option in the settings dialog of the gem manager seems to work on an all or nothing basis, even disregarding what list I look at. So even the list of updated gems, which currently only shows one entry, will produce the error when updated. Feature does work properly on local i.e. installed gems. 

I suggest a feature to narrow down the list of gems fetch the descripton for or maybe fetch the description for selected gems. Best solution in my opinion were an option to auto-fetch the description on selecting a gem on the list. This could replace the "Fetch Detailed Description" option as this doesn't work anyhow. I also doubt that this error will be fixed in the ruby gem tool any soon, as hardly anyone has the need to fetch the description of currently almost 16000 gems at once.