Just to be clear, you are getting this issue with every single node in your setup?
Do the devices pass the SNMP test? Go to Manage Nodes, select a node and click on Edit Properties. Then half way down the page there's a Test button.
I was having this same issue after upgrading from NPM 11.0.1 to NPM 11.5. I tried added new nodes and it would spin, I checked SNMP on existing services and WMI, they were both successfully but then on the next screen the List Resources would never complete.
For me it turns out that after the update my Job Engine v2 service wouldn't start. it would start but would crash. After doing some search I found this article Solarwinds Job Scheduler will not start that had me replace my JobEngine32 database with a blank one (also located in the same folder). After do that the Job Engine v2 service started and I could list resources again. The only thing I'm unsure is of the impact that replacing that file had as the previous file was a lot larger so it had something in it? I opened a support ticket to ask them what that impact was. I will reply to this once they confirm.