I have 9 netVanta 1550 48-port switches. Very basic setup with 2 VLANS. After booting them, while telnetted in, there are no errors. However within 2-3 days I am unable to log into them (SSL, Telnet, or management cable) and if you plug a device into any unused port, there is no connectivity. However, all ports that were active before the interface failed still work. Rebooting the device returns everything to normal. The CPU load rarely exceeds 50% and the PoE load averages around 25%.
This sounds very much like the Firmware has not been updated at the installation to the recommended and supported R13.5.1.