My Network Management Card (NMC) is losing SNMPv3 connectivity after a certain amount of time but other communication protocols (web, FTP, SSH, Telnet, SNMPv1, etc) continue to work properly.
Update (July, 2016): Specific to NMC1 products, this issue had previously been fixed for AOS 3.7.3 and higher. The maximum SNMPv3 uptime allowed was modfied from 49.7 days to 497 days. At the 497 uptime day mark, assuming the management uptime had reached this point, the Network Management Card interface would have had to be rebooted though the web, CLI, or SNMPv1 interfaces in order to regain SNMPv3 communication. NMC2 AOS v5.1.X also contained a fix to avoid an SNMPv3 communication loss completely.
With NMC1 AOS v3.9.2 and higher, this issue has been fully resolved and the 497 uptime limitation has been lifted and SNMPv3 communication through StruxureWare Data Center Expert or any third party SNMPv3 NMS will not be affected after 497 days of uptime. This AOS (or newer) will be released for different firmware applications in the coming months which are available at http://www.apc.com/tt/en/tools/download/. Certain applications are available immediately, such as Rack PDU.