Try selecting a lesser number of nodes, then try to change the pollers. You may be hitting the SQL response time limit which is why you're seeing the SWIS error. If you're trying to edit all your nodes at once, this is likely your problem.
Having an issue of a similar type just today in manage pollers, but only when I click "Assign" and go to group by Scan Result. Any other group by clause works.. When I choose Scan Result, I get "A query to the Solarwinds Information Service Failed". Any ideas? I will be going through the web logs to see if I can figure it out.