Glue: Testing connection triggers "Task allocated capacity exceeded limit" error

We have a Glue connection that was running successfully. Now, though, even when testing the connection we get a "Task allocated capacity exceeded limit" error. We have checked the max DPUs and concurrency quotas. They're fine. The connection is going though a VPC, but that was working a couple of days ago. No CloudWatch logs are being written, so it's impossible to know what service / quota limit is triggering the error.