On my UC300, a similar problem, here's what I found out:
- Error 500 appeared after updating the firmware to version 3.x. Fortunately, it does not appear on all pages (it was noticed, for example, on the PBX/Extensions page). After rolling back to firmware version 2.5.6, the error was resolved.
- It is not possible to connect to UC300 with the putty - an error "Connection refused" (both in telnet mode and in ssh mode).
- Reboot does not help.
- Pop-up messages say that updating from version 2.x to version 3.x needs to be done through version 2.5.7, but there is nowhere to download it.