Redmine

For me the documentation works pretty well. There was only one point where I stumbled upon initially:

5. Copy the folders of your custom plugins from your plugins directory ... Make sure that you copy only plugins that are not initially bundled with your previous Redmine setup.

Here lies the problem, I had copied the old plugins bluntly and so the errors occurred because individual plugins were already bundled. That's why I've got used to not copying all plugins anymore, but always reloading the current version from the source - there has never been any loss of data.