Error when trying to increase storage on RDS MySQL instance

I'm trying to increase the allocated storage from 2000GB to 2260GB (and IOPS from 6000 to 7000) on an RDS MySQL instance and I'm getting the following error message:

null (Service: AmazonRDS; Status Code: 500; Error Code: InternalFailure; Request ID: ea593451-3454-11e5-bc38-b7fa8a060cf1)

The read replica for this instance has had it's storage and IOPS increased, so that's not the issue.

Any ideas what might be causing this? If I've missed any key info, please let me know in the comments.