ServiceStack.Redis deserialization issue - sometimes JSON is corrupted

Question

I got a service that uses ServiceStack.Redis for storing objects (serialized with JSON). There's a key that's updated with each HTTP request - the flow is simple: get value for the key, deserialize it to a list, add/remove items, write modified list back to Redis.
Recently, we noticed a problem - some of the keys will become corrupt (so they can't be deserialized). I don't know why.
Exception says Type definitions should start with a '{', expecting seralized type X, got string starting with: Y at ServiceStack.Text.Common.DeserializeTypeRefJson.StringToType(Type type, String strType, EmptyCtorDelegate ctorFn, Dictionary'2 typeAccessorMap.
I read other answers - most of them imply you get JSON data from another computer or you don't have control over how it was built. But I do.
We use only few types - most of the properties are simple ints, others are DateTime.
I have only one theory - multithreading is involved. But in that case, it should be "last-write-wins" and no corruption should occur. Anybody else saw that behaviour?
We're using ServiceStack 3.9.71 (latest release from 3.x series) with Redis 2.8.9. .NET 4.5 web application is hosted on Windows and Redis server is on Linux.