I have tried updating my 8.5.5 site to 9.0.0 which produces several errors.
As I probably want to reorganize my site anyways, is there a way to install a fresh 9.0.0 version and then import the old content (including files and images, blocks etc.) from a previous version?
We had something similar with the migration tool once.
There’s not really an exporter/importer as such for content like that.
But unlike the transition for 5.6 to version 5.7/V8, there shouldn’t be a need to go to those lengths.
What I’d suggest is just holding off doing such an upgrade for a while - let things settle with V9, for some bug fix releases, etc. Whilst V9 is great, there’s no urgency as such to do an update just because it’s newly out.
Then when you do try and upgrade again in the future, it might either work, or you can return here with some specific error messages.