This error indicates that Dataflow couldn't locate a default subnet within your network in the europe-west3 region. Dataflow requires a subnet to place worker VMs and manage network traffic efficiently. This broader error signifies that GCE might have been temporarily unavailable. 

The absence of a subnet in the europe-west3 region directly caused the first error. Even without explicit network settings, Dataflow requires a subnet to operate.

The second error suggests issues with the europe-west3 region itself. Even though a new subnet appeared and might seem to resolve the issue, persistent problems with Compute Engine could still be causing the UNAVAILABLE error.

Here are some troubleshooting steps to address:

Network and Subnet Verification

• Confirm Subnet Existence: Ensure that your default network now definitely includes a subnet in the europe-west3 region.

• Subnet Configuration: Verify that the subnet’s IP range is sufficiently large to accommodate the worker VMs that Dataflow will create.

• Permissions: Check that the Dataflow service account has the necessary permissions (Compute Network User and Compute Instance Admin) to operate within the subnet.

Try an Alternative Region

• Temporary Workaround: If persistent UNAVAILABLE errors occur, consider executing your Dataflow job in a different region to determine if the issue is specific to europe-west3. Note that changing regions can affect data residency and transfer costs.

Inspect Compute Engine Quotas

• VM Instances: Review whether you are nearing any Compute Engine quotas, particularly for the number of VM instances in europe-west3. Quota information can be found in the Google Cloud Console, where you can also request increases if necessary.

Advanced Troubleshooting

• Firewall Rules: Confirm that your firewall rules permit necessary internal TCP and UDP traffic for Dataflow worker VMs, allowing both inbound and outbound connections.

• Metadata Limits: If your pipeline uses a significant number of staged JAR files or complex options, consider simplifying these to avoid hitting metadata limits.

Additional Considerations

• Explicit Network Specification: For better control and to avoid reliance on default settings, explicitly specify your network and subnet in your Dataflow pipeline options. Refer to the Dataflow documentation for instructions on how to set these parameters.