That because of i'm confused by the available documentation and my experience with that in past (means with 8.1.x).
The documentation Upgrade to 10.0 says:
We recommend that you keep your ownCloud server upgraded regularly installing all point releases and major releases, without skipping any of them, as doing so increases the risk of errors.
Before beginning an upgrade, please keep the following points in mind:
Review the release notes for important information about the needed migration steps during that upgrade to help ensure a smooth upgrade process.
A view in the Release notes shows that for every version the notes different issues are mentioned, that means for me, starting with 8.1, i have to pay attention to every release note for every major version.
It is now possible to directly upgrade from 8.2.11 to 10.0.3 in a single upgrade process.
is mentioned for 10.0.3 and not for 10.0.4, ok.
After upgrading to ownCloud 9.0 and before continuing to upgrade to 9.1 make sure that all of your and your users Calendars and Addressbooks are migrated correctly. Especially when using the IMAP user backend (other user backends might be also affected) you need to manually run the mentioned occ migration commands described above.
and if i go back to Upgrade to 10.0
Skipping major releases is not supported. However you can migrate from 9.0.9 straight to 10.0.
That means what? What is with 8.1.x as mentioned at the other documentation?
In knowledge of major changes with the contacts and the the release notes for 9.1
Removed dav:migrate-calendars and dav:migrate-addressbooks commands for occ. Users planning to upgrade from ownCloud 9.0 or below to ownCloud 9.1 needs to make sure that their calendars and address books are correctly migrated before continuing to upgrade to 9.1.
i decided to go step by step thru the major versions. And with my understanding of all that kind of informationen i am sure, that is a right way. I don't want to to loose any kind of data during a upgrade.
My problem is, that major changes are made and is quiet hard to follow from one stable version to another.
And an other issues wich is important is php 7 and change of subsystem in Ubuntu. Ubuntu 14.04 LTS is in my opinion not able to run php 7 and will reach end of life soon. So i want to change with last update to 9.1 the server respective the os to Ubuntu 16.04 with php 7 and the change to an other subsystem. So keeping that in mind, the best way will be go step by step thru all major versions.
normally doing such procedures is not really a problem for me, but in may lack of core knowledge of owncloud and the big warnings about loss of data in the release note i am a bit carefully in doing things in the installation.
So, before i step to to upgrade to 9.1 its for me important to fix the contacts issue.