I'm also getting the same issue. I'm trying to create a E2 workspace using Terraform with Customer-managed VPC in us-east-1 (using private subnets for 1a and 1b). We have 1 network rule attached to our subnets that looks like this:  

Similar question here. How can I debug this? Would it help if I gave you the "get network 123-456" id?

Things work fine if I used the Databricks-managed VPC example from the docs. It also works fine if I use a ***** VPC with */* all allow.