Why does Synology Active Backup for Business show my PC as offline when it is switched on?

Applies to: Active Backup for Business agents on supported Windows PCs or servers. This is a connection check, separate from VSS backup errors.

Work through these checks

1. Check whether the PC is awake and connected to the expected LAN or VPN. Open the agent on the PC and record its server address and exact connection message.
2. From that PC, verify access to the intended NAS address. Check changed DNS records, NAS IPs, VPN routes and firewall rules. Active Backup agent communication normally uses TCP 5510; allow only the required trusted path.
3. Check that Active Backup for Business is running on the NAS and that the client agent service is running. Compare NAS and PC dates and times.
4. If the NAS address or certificate changed, verify the server identity and reconnect through the agent using an authorised backup account. Do not blindly accept an unexpected certificate or expose the backup service publicly.

Confirm the result

The device should become connected in ABB and complete a small scheduled or authorised test backup. Then confirm the resulting recovery point is available.

If you need help

Include these details in your ticket: DSM/ABB/agent versions, agent status, server address type, LAN/VPN context and redacted error screenshots.

Updated 261107