I'm getting an error running

pulumi up

in a situation where there should be no changes:

Diagnostics:
  aws:elasticache:Cluster (example):
    error: unmarshaling urn:pulumi:dev::core::aws:elasticache/cluster:Cluster::example's instance state: could not read field snapshot_arns: '' expected type 'string', got unconvertible type '[]interface {}', value: '[]'

That seems like you’re giving that the wrong type, from what I can see there it’s expecting a string and it didn’t get it. You might get more help putting in the code for the resource you’re creating and not just the error message.

This script hasn't changed for weeks and was working fine last time I ran it, so I don't think it is the code...

That seems to be the problem. That thread has a workaround, though it should work for you with the initial version you used before. So rollback to that version, or use the workaround in that thread. Apparently there’s a breaking change. Nice catch.