When we switched from Jackson version 2.8.9 to 2.9.8 we ran into some trouble with serializing data in CBOR format. As it turns out the custom Jackson module we've been using is causing trouble in this newer version. These issues start at version 2.9.0.
I've made a repository that reproduces the issue and found that when I disabled our custom TemporalAmountModule issues went away. I did not write the TemporalAmountModule myself but it looks like the Custom Serializer and Deserializer are the cullprit. These do not introduce any complex code but I don't really understand why they don't work properly anymore because I am not to familiar with Serializers and Deserializers.
I've made a repository that reproduces the issue I am having. When you disable the module you can see it is working properly. When you change the jackson version to 2.8.x (latest in that range is 2.8.11) you can see it works properly too.
https://github.com/Archcry/reproduce-jackson-error
The expected result should be the same as it was in version 2.8.9 (or 2.8.11 for that matter). Jackson should not break any functionality in a minor version release but it may deprecate certain methods (which they did). The deprecated methods are not the ones causing the issues though because I tried updating them but ended up with the same result. The actual result at the moment is an error saying Can not write a field name, expecting a value.
Issue was solved on GitHub, see https://github.com/FasterXML/jackson-databind/issues/2320
Edit: Issue is being worked on and shouldn't have happened in the first place.
Edit 2: Issue is no longer worked on, due to the lack of context it's hard to reproduce.
Unfortunately while I can see the exception from sample project, I am not able to construct simpler version with combinations. I wonder if that might be related to use of immutables, annotations. So I think I'll leave this as-is for now.