Value (us-east-1c) for parameter availabilityZone is invalid. Subnets can currently only be created in the following availability zones: us-east-1b, us-east-1d, us-east-1a, us-east-1e.
I tried manually creating a subnet in us-east-1c in this account, and AWS threw the same error again. I'm not sure of the cause on the AWS site currently, but it didn't happen when I tested in another AWS account.
If anyone encounters anything similar in their own setups, I was able to avoid the problem as follows:
Download the template, and edit the following section:
"Mappings" : {
"SubnetConfig" : {
"Subnet1" : { "AZ" : "us-east-1c" },
"Subnet2" : { "AZ" : "us-east-1d" },
"Subnet3" : { "AZ" : "us-east-1e" }
},
In my case, I changed us-east-1c to us-east-1b, then deployed with the edited template instead, and it deployed without any further problems.