Just in case anybody is interested, after working with Cisco we have a temporary fix for this. The problem started happening after the Radius (AD) account password was changed.
I tried the following to fix it, remove the Fabric’s and re-discovered them with the new password but that failed. Re-installed DCNM and tried Radius Vs Local but neither worked.
The CLI still worked but both Fabric & Device Manager failed, after running the hidden command “
sync-snmp-password”. It worked for both FM & DM for Radius but it seems that I might have to do this every month when my password expires. Hopefully a full fix will be released in the future.