When updating from a prior version to Summer 20 H2 you might have to make changes to the user that runs the container. If you set it to www-data in the past, you have to change it to apache.
Please note, that you might also need to change the owner of the /opt/aagm folder to apache:apache. If this is the case, the Gateway Manager will issue a “folder not writable” warning when booting up.