Unable to locate the xml-definition for FieldName with FieldId ...

If you're trying to deploy some custom site columns using a feature and the feature activates fine, but the columns don't show up and your logs give you the above error, then the solution is pretty simple: when you're defining your columns, you have to make sure to use brackets around your column guid, like this: {GUID}. Deploy your feature again, deactivate it, activate it again and everything should work fine! You also have to use the brackets when you're referencing the column in a content type!

Thanks to Edwin Vriethoff for posting this!