As it is essential for data security to save my current calendar and contacts to ics and vcf files I did some more testing.
Thanks for feedback about the URLs. The contacts URL at least did not really change but turned out to work under 9.1.4 in two flavours:
Under OC 10.0, however, only the first version works correctly.
It turns out, that newly created calendars can be saved without problems. As a workaround I created a new calendar and imported the latest (right before the backup) ics file. The import, however, hangs without error message. While importing I could further use OC from the browser and when back to the calendar the import dialog has disappeared. I straight exported the calendar to a file and compared the imported and exported files: Missing about 150 VEVENTS.
To make tests complete: An import of my contacts failed as well. This time, however, an error appeared: Multiple PRODID entries makes the importer refuse to work. Although I could solve this be deleting ALL PRODID entries I did no further tests.
As I spent meanwhile hours of testing I decided to to a manual downgrade to the previous OC 9.1.4 port, which in fact was a reinstallation with a subsequent import of my contacts and calendar file. The import of both my contacts and my calendar went just fine under OC 9.1.4.
Summary: I have full understanding that a major release upgrade reveals several new errors and smaller issues. However, I cannot understand why basic functionality is obviously not tested at all BEFORE releasing OC 10.0. At least two severe errors could have been easily detected before:
1.) Missing calendar due to the initially described wrong conversion of at least one table.
2.) Broken import of contacts and calendars.
I am observing now these severe problems with both calendar and contacts apps with major release upgrades and it every time took me many hours to get my data saved and find a workaround. For me and obviously many others these two apps are and essential part of OC which is expected to work in a reliable way - even after a major release upgrade. But obviously there has never been focus on these two apps under OC. I am tired fearing data loss with every major release upgrade and seriously take into account of migrating away to an alternative with focus on these two apps.
Unfortunately, I cannot even write a helpful Github issue on my observations, because there are no or only very limited error messages which are not helpful for developers. So I hope, this feedback can be helpful instead.