I want to set an issue's resolution to "unresolved" using the REST API, but I don't know how.
I'm able to set the resolution to any resolved state using a PUT request on the affected issue with a body like this:
{
"fields": {
"resolution": { "name": "Done" }
}
}But if I try to reset the resolution using a body like this:
{
"fields": {
"resolution": null
}
}The the request fails with the error message:
Could not find valid 'ID' or 'name' in resolution object
Question: How to reset the resolution field to unresolved using the REST API?
Background
Why do I want to do it? Well, we have a bunch of workflows looking like this:

It is easy here to set the resolution (in the transition to "Gelöst" or the direct transition to "Geschlossen"), but the resolutions needs to be reset in nearly every of the other transitions. As this is only one of 13 workflows in our system and each of them contains about 7 transitions which require to reset the resolution I would need to add a postfunction to reset the resolution manually to about 7*13 ~ 90 transitions.
Thus I decided to implement a script listener using ScriptRunner for cloud which resets the resolution to unresolved on every issue update if the affected issue is not in one of the resolved states (in the example above these are "Gelöst" and "Geschlossen").
Unfortunately this doesn't work at the moment as I don't know how to reset the resolution.