Openhab site can't be accessed after backup

[Matze0211 (Matthias)]

Restoring a backup created by a different version is not supported.

There are multiple posts here and a lot of people face similar issues.
Searching the forum will lead you eg to the following

openHAB 3.2 Release discussion

The file org.apache.karaf.features.xml was functionally empty: ’ <?xml version="1.0" encoding="UTF-8"?> <!-- Licensed to the Apache Software Foundation (ASF) under one or more contributor license agreements. See the NOTICE file distributed with this work for additional information regarding copyright ownership. The ASF licenses this file to You under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You …

If you want to do it correct please use the following order:
Install OH 3.0.2
Restore the backup
Upgrade your system to latest version