Migrating ownCloud 10.4.1 to nextcloud 18 Odyssee

Migrating ownCloud to nextcloud typically isn't that hard thank to the nextcloud developers who provide a user friendly migration script: https://nextcloud.com/migration/

Sadly at the moment the migration script only supports ownCloud migrations up to 10.0.x and not 10.x.

1) Old PHP Versions

First you have to keep in mind that upgrading ownCloud 10.x to 12.x requires < PHP 7.2 (NOT 7.2.x as the error message might tell you), so I'd suggest to use 7.1.x. The reason is, that ownCloud 10.x migrates to nextcloud 12.x. Afterwards you step by step upgrade nextcloud to the most recent release (18.x currently) and increase the PHP version at 17.x to 7.3 or 7.4.

2) "The following extra files have been found..."

But there was further trouble I ran into. First I encountered the error:

"Check for expected files The following extra files have been found: ..."

where I could find a solution here:
https://github.com/nextcloud/server/issues/13568

Next error was:

3) "Download failed - HTTP status code: 429 ..."

"Download failed - HTTP status code: 429 - URL: https://download.nextcloud.com/server/releases/nextcloud-12.0.12.zip "

Which I could solve as suggested here by commenting out the download code and uploading the .zip file by FTP before the next run: https://help.nextcloud.com/t/error-while-downloading-http-status-429/72084

4) "Updates between multiple major versions and downgrades are unsupported"

Finally the hardest problem was:

"Updates between multiple major versions and downgrades are unsupported"

(https://help.nextcloud.com/t/updates-between-multiple-major-versions-are...) after I thought the problems were solved. Running the occ upgrade wasn't possible though.

So I tried something dangerous, but it worked for me:
Commenting out the line which triggered this error in the compatibility / version check, as suggested here. That might break things, but was worth a try... and yes, the update continued!

5) "An exception occurred while executing..."

But things still didn't work! Finally I had to delete indexed from the oc_persistent_locks table like described here due to a new error now:

"An exception occurred while executing 'DROP TABLE oc_accounts':
1217 Cannot delete or update a parent row: a foreign key constraint fails"

https://github.com/nextcloud/server/issues/14549

As it was empty, I decided to delete it, recreate it without the affected indexes and went on with the update.

6) Violla! UI works! Cleanup.

Afterwards I went to the administration page in nextcloud and solved the minor issues like this:
https://docs.nextcloud.com/server/18/admin_manual/configuration_database...
https://docs.nextcloud.com/server/18/admin_manual/installation/server_tu...
https://docs.nextcloud.com/server/18/admin_manual/installation/harden_se...
and missing indexes.

Wonderful UI and features nextcloud! Great work :)
I guess I hit nearly all kinds of problems possible when migrating to nextcloud ;)