Spot fleet request stuck in "cancelled" state

I have a spot fleet request (sfr-7af1849d-baf5-4de1-af14-7188fd3ea76b) that has been sitting in the "cancelled" state for several hours, with no instances running. This is preventing me from modifying the target capacity of this fleet request in order to bring on additional instances.

In case it matters, this Spot Fleet was created as part of an ECS cluster. If I attempt to scale the container instance count from the ECS cluster I get this error:

The Spot Fleet request can't be updated in the CloudFormation stack. Wait a few minutes and try your request again.

Stack:arn:aws:cloudformation:... is in UPDATE_ROLLBACK_FAILED state and can not be updated. (Service: AmazonCloudFormation; Status Code: 400; Error Code: ValidationError; Request ID: 8e38038f-1f24-11e9-bcd5-db795b8594f7)

Any help would be appreciated.

Thanks!